I'll probably just make sure I got all of the basics covered, which shouldn't be hard since some seem to be pre-made. Christmas, Easter, and Thanksgiving are definites. Mother and Father's Day must be included as well. July 4th/Independence Day is another must.
I'll probably have New Years too, and I guess Valentine's Day. As for others, I also think it would be cool to incorporate foreign holidays such as Japan's Children's day since all of my families have kids.
I'm not sure what holidays I'd create. I might make a special holiday for each season so I can make sure there are festivities year round. Maybe Autum could have a "Leaf Day" where kids play in leaf piles and parents rake them up afterwards :D Idk, but I'm having fun think about it.