I want to add Solstices as well as all the traditional holidays. The more celebrations, the better!
Valentines (Romance Day): romantic presents, romantic restaurants, spa days, date night.
St Patricks Day: Decorating everything in green.
Easter: Egg hunt, buying plush bunny toys. Feast brunch. Possibly other interactions with custom content.
April Fools Day: Dedicated to jokes and pranks, and perhaps grilled cheese.
Spring Solstice (May Day): Dancing around a May Pole object (probably custom content), burning floral incense, gathering flowers and making bouquets.
Summer Solstice (Picnic Day): Summer picnic and BBQ in the park, bonfires, hot dog and marshmellow roast. If they add watermelon, definitely eating watermelon on this day.
Halloween: Pumpkin carving, costume parties.
Fall Solstice: (Feast Day or Thanksgiving) Harvest feast, Fallen leaves themed activities, roller skating, Baking of bread.
Winter Solstice: (Snow Day) Snow and ice themed activities and decorating with holiday lights. Dance parties.
Christmas: Tree decorating, gifts. Possibly other interactions with custom content.