Pidgeys, Weedles and Caterpies

You can save them if you want, but seeing as they are so common it's not going to be hard to catch more before we might get a double XP event.
So I would to a mass evolution.

Also, we don't even know if/when we'll get an event with double XP. It might happen soon or it might happen in like 6 months or more.
So it might be frustrating to wait.

lol maybe just lucky egg it? I can't see them having the same holiday event easter if anyone is still playing by then seems like a egg special event I'm guessing.

I'm in the same predicament. Do what I'm doing...I'm only doing mass evolutions ONLY when I'm starting to lose Pokemon space. That way, you can still progress and still have Pokemon to evolve later. I'm hoarding until another double XP event happens. Hope that helps :)

I have over 100 Eevee's ready to evolve with enough candy to do it. I also have candy to evolve over 100 "twelvers" but I've been letting them go due to space shortages. So I'm kind of in the same predicament. For people who are under level 30 I think it is best to use them now but in your case I would wait so you can get to level 40 faster. Although, I know it would be nice to get level 36 faster at the moment.

I wouldn't work on the assumption that any particular event is going to be repeated. They're great to have but are an extra bonus and shouldn't influence long term game strategy.

If there is one thing we learned so far from. iantic is that we can't predict what the events will be and they don't repeat events (at least not yet).

I wouldn't wait for a double XP event because it may never happen again...