If you abandon a race -> „N“ over „A“, if no race is running -> „AP“ over „A“. See „RACE SIGNALS“ . Both options have only the meaning for this day. 

No matter what may be planned for a later date, both AP over A and N over A just signal that there are "no more races today". 
AP (over A) concerns only boats that are not in a race yet, i.e. that have not been started. N (over A) is displayed when at the time of the decision a race is underway and has to be abandoned. 
If, at the time of the decision to finish for the day (e.g. due to a storm coming up), some boat classes are in a race while others are waiting for a start procedure, I would first dieplay N to abandon the race underway, then add AP over A. 

Another option is to use AP or N over H instead of A.

I think there's a pretty big difference between the two. Race Signal N over A is defined starting with "All races are abandoned." All means all, and the RC can abandon any races pretty much at any time (RRS 32.1) once they've started, altho the RC must "consider the consequences" abandoning a race will have on all boats if abandoned after it is scored. I have no idea what that means in terms of guidance, but I have had cause to consider it in the past - long story and will cost you a beer. In any event, N over A (or H, for that matter) means at least one race that has started is being abandoned, scored or not. Race Signal AP over A (or H, for that matter) starts with all "Races NOT started are postponed" - those that have been started and scored are in the books. Either flag over A means we're done for the day. The big difference lies in what we do with the races that have started, and we have to consider what to do with them if they are scored, in the one instance, but not the other.

To answer the cases presented above:
a) If races not sailed in an event during the scheduled time in the NoR or SI's without a provision for rescheduling them (see RRS J2.2(27), I think you're out of luck and the event is not sailed. Often a distance race, for example, will have such a provision in the NoR.
b) If there is another day to race after a race is missed and there is provision to make one up, either explicitly or implicitly (e.g. 2 races scheduled for a day, no more than 3 may be sailed in any one day) then you're good. You can always change the schedule per the SI for doing so to facilitate the sailing of an additional race anyway.
c) If the last day has any opportunity to sail at all, yay! If in the days prior to the last you have met the requirements in RRS J1.3(5) for number of races to constitute a series, if you don't get any more in such is life (and don't get into qualifying for a discard with me!). AP/A is appropriate, there just isn't a day in the schedule (unless provided per the answer in a above) to sail the races postponed. If you haven't made a series as yet, Lady Luck is just not on your side this day. I always put in a provision in the prizes that the OA can award any other prizes at its discretion. You may not be able to award an intergalactic champion, but you can acknowledge whatever racing did take place. I would rather take the heat for not getting a regatta in than take heat for the regatta having awful sailing conditions and the racing was very low quality. Nobody likes a crapshoot.

Fair winds!

Mianne,
N over A will abandon all races, not only the races started; all of them,  started, and yet to start.
No need to double signal with N first, then AP,  fleet can be confused.
I'll be confused myself: the races started are abandoned, (or all ther aces are abandoned) but the races yet to start, are postponed? Raise N over A, all boats to go home, period.

Mind not confusing N and AP (over A or H):  N  = abandon all races;  AP =  postpones the races not started. 

Personal suggestion: N or AP raised for safety reasons by RC are not "mandatory", as far i understand from the RRS. No more races, more signal ashore, whatever, but no "obligation" for the boat to return to harbour, or any safe place.
In case of a serious incident, minding the responsibility of the RC or OA, notwithstanding the assumption that all risks lay in the hand of the sailor, i'd not be, personally, very keen to explain that to a penal court.
So, in the SI, i'd insist in making a modification to the meaning of the signals, adding that the meaning of N over A or H or AP over A or H,all boats are requested to return to harbour or any safe place the nearest. And with a standard penalty just to make it remembered. That will force boats to return, and not to keep "excercising" or beating around the bush, for the fun of it, or to prove the RC decision was wrong, look at me: 40 knots and going with the spi; look ma, no hands...

The mnemonic I always use with my RM students is, "You can't postpone a race that's underway and you can't abandon a race that hasn't started" (although, technically, you can).

US Sailing has a prescription to Appendix LG (SI guide) that the display of Alpha at the end of the race with no sound means, "No more racing today."  This tends to work reasonably well.

If I have a race underway, I will use N over H; rarely N over A unless it's the last day of an event.  Otherwise, it's AP over H or AP over Alpha.

The problem with N over A or AP over A is that it's the "nuclear option" - you're done for the day, no matter what.

What gets me about all this is that pretty much all race signals are the official means by which we communicate with the sailors, our customers. The rules for doing so are pretty clear, and if those rules are not adhered to there could be confusion among the customers, or, worse yet, grounds for redress. Following the rules does not preclude further and/or expansive discussion of what we're doing, it's just that everybody has to get the same information pretty much at the same time (hence sound signals to draw attention). Here, anything with an AP means stuff that hasn't started won't for at least a little while (staying within the NoR schedule if and as amended according to (!) the rules). Likewise anything with an N means that something that HAS started is being pitched out. Different signals using either AP (4 different ones) or N (3 different ones) have that common thread between them. And sometimes the weather gods (or general luck) just isn't on our side that day/weekend/week, and we all get skunked. As much as we may not like it, sometimes we get reminded who's Boss.

Oh, and Matt, I can't find the A flag Rx in the new USSA appendix LG. It was 5.6 in last quad's Appendix L, but it's not there now in the PDF version on their site. Since this thread was started by an Italian it probably isn't relevant anyway, but if 3 races are scheduled in a day, and 3 are completed, do we really need to signal anything? And if there isn't a daily schedule (just a max number for the series), is just A that much different than AP/A?

Fair winds!

See US Sailing Prescription to RRS 32:
"After rule 32 add:
US Sailing prescribes that, if the sailing instructions so state, the race committee may display flag A (with no sound) while boats are finishing to signal that there will be no more races conducted that day."
In case (c), this seems more appropriate than AP/A because races are not postponed since it is the last day of racing.

That Matt, such a mis-director.  In any event, I don't think either method (A or AP/A) is incorrect. Personally, I prefer to use what's in everybody's book, not just ours. SIs are long enough as it is. YMMV.

Fair winds!

I avoid using Code Flag A on its own as it means "I have a diver down; keep well clear at slow speed." 

I avoid using N over A as you have to wait until all boats have finished before it can be displayed, by which time most of the boats have already asked you if there is any more racing and are headed for the dock.

If there is a time in the Notice of Race (or sailing instructions) after which you cannot have a warning signal and it has passed, I just pull up the anchor and follow the boats in. They have already figured out there is no more racing.

If there is no time in the Notice of Race (or sailing instructions) after which you cannot have a warning signal and you have more scheduled races, you need to try to run the races.

I thanks all of you.
Meanwhile I received a massage with a link to a 2015 Q&A (thanks Gianluca!) that explain the situation, expecially at the question nr. 2.☒

Stefano - can you provide the link itself? I don't know where to search for the file.  Thanks!

Fair winds!

Most cool, thank you! Unfortunately these things aren't widely circulated (because they don't have "rule" status?), but fortunately we have this to help us help each other!

Will confess to disagreeing with Answer 3, as a completed (i.e. scored) race can indeed be abandoned. Very helpful anyway!

Fair winds!

N flag displacement is not ment for abandoning scored races. 

The Q&A discusses displaying N over A ashore. Displaying flag N does not abandon completed races. 

Here is a link to a number of World Sailing Question and Answer Booklets which a free to download https://sayra-sailing.membershiptoolkit.com/story/328367

Thanks