Golden Apples for 1/2 Daily or for events?

It comes down to what you need more and how many apples you have. If you have servants that you regularly use or want to use that aren't max level, you should probably level them. If there are no servants you want to level or only a few then you can go ahead and save them. Just to let you know though, the 3x rate on top of the 1/2 ap is a godsend to level any servants. If there is any servant that you think you might want to use in the future, you should use this opportunity to level them. In the first day alone of this event I have leveled 2 5* and and a 4* from level 1 to max level using only 5 or so apples. I personally think its worth throwing a few apples at. Just make sure to save a few for shops.

just use apple for whatever you want. every event give you some apples anyway

Whenever AP events like this happen, I usually budget out like 4-5 apples for farming XP, since it's so useful in the long run. I like to have a stockpile for big events (I have no idea how many I used during KnK, but it was at least 15) but that doesn't mean I don't allow myself to use some here and there.