Reference: Calendar & Holidays

Spring

Valentine's Day is celebrated on the third day of Spring.  Parents' Day is celebrated on the fifth day, but these are not work or school holidays.

Summer

Children have the summer off from school (thanks to Phaenoh Summer Vacation Winter's Day Package, here.  (Also the fourth day of winter is a holiday from school with this mod).   Parents still have to work during summer, so many kids have to go to "day care" during that time if no one can be home with them.  Also, if kids need to bring up their grades, they can go to school, but the bus does not come.  

Fall

The fourth day of Fall is Thanksgiving.  Usually extended families get together for a feast on this day.  Some families may celebrate this on a weekend if they are not able to get off from work for the day.

Winter

As mentioned above, the fourth day of winter is Winter Holiday.  Some sims celebrate Christmas, Kwanzaa, or Hanukkah on this day, but children do not have to attend school and most sims have a work holiday.