Thinking about this idea, I don't think it would work because a) it would be confusing for some people XD and b) I think the creatu would have to be released bi-annually or something. :'x   

The summer solstice occurs in June in the Northern Hemisphere and in December in the Southern Hemisphere. The winter solstice occurs in December in the Northern Hemisphere and in June in the Southern Hemisphere.

We already have a seasonal creatu that comes out around the time of the solstice that occurs in December.

And call me picky but if you're gonna do one solstice, you have to do the other AND the two equinoxes as well. XD

I don't support for Fourth of July, since we have users from all over the world and it really wouldn't be fair.  Though I do believe we should have Res Day, to celebrate the day Res was founded.  That seems kinda new-creatu worthy.  :3

Solstices would be pretty cool, though. 

4th of July is American Independence Day.

^See what I mean? 

There are plenty of new things in the works that aren't related to any holiday ;]