This is my official recommendation. Since I am a male I would take this with a grain of salt but then again take it seriously because you could seriously benefit from this.
Niantic has been consistent with American Holidays celebrating Halloween and Thanksgiving. We saw some awesome events and hopefully Niantic will come through for future events (which we suspect to be the case). The next big holiday? Christmas! AND New Year.
If you are under level 30, or are a casual player, you have no reason to evolve your Pokemon after the Thanksgiving event. To make this clear: Definitely evolve during the Thanksgiving event because you’re gaining 4x experience with a lucky egg right now, but after said event you should stock up because we do not know what the next event will have in store.
On the other hand, we could argue that Niantic will attempt something entirely new with the next event irrelevant to exp gain. We recently observed their efforts in Japan to mass-spawn Lapras and it would be a nice present if the rest of the world received the same reward, but either way you are not taking a risk.
It is widely known that level 30 and above observe the same Pokemon. In other words, whether you are level 30, 33, or 40, wild Pokemon’s CP are capped. The only benefit to level beyond level 30 is to theoretically power-up your Pokemon (which from level 30 to level 40 would require around 175,000 stardust for a 15% CP increase). In a watered-down combat system, it is not really much of an incentive as of now.
“Ej, get to the point you thick-headed little Italian-bearded man.” Okay, Okay. What I am saying is you lose nothing and potentially gain a lot. I am uncertain of what the next event will be and when I receive word (hopefully from Niantic) then you can bet I will write about it, but this is a no-risk investment. So, until we find out more, it is better to save your Pokemon in case there is another experience-related event.