If I get Sims 2 Holiday Pack, how will it work with Seasons?

I already have Sims 2 Seasons, and I'm thinking that I'm getting Holiday Stuff for Christmas :].

I'm wondering, if I install these two games together, how will they effect each other during gameplay? Will Christmas be in winter, Halloween in fall, Easter in spring, etc? Or do you get to choose when they celebrate each holiday?

Thanks xD
Merry Christmas!!