There is another thread
here that I posted in already, but I'll copy/paste what I put in that one:
Earth Day - I'll have my Sims plant trees on their lots and/or travel to all the different parks and plant one there.
Elder Appreciation Day - My take on Grandparents Day only for all elders. Give gifts to random elders on the street, in parks, etc. Have a party for any elders in my families and/or take them out to a restaurant.
Purple Day (My favorite color) - Everybody will dress in purple clothing, homes will be decorated in purple objects and maybe only eat purple food. Might forgo that last part since they'd end up eating mostly lots of grapes and blackberries. lol
Freezer Bunny Day - Absolutely LOVE that idea!
Llama Day - Why let the Bunny have all the fun? Llamas are important too eh?
Plant Sim Day - You guessed it, everybody in all of Simland will be a Plant Sim for the day and the Sims I control will only eat vegetarian dishes.
Kids Day - Spend the entire day doing things with your kids, take them to the park, read them stories, play games with them, give them gifts, watch movies with them, have cake & ice cream, etc. Could include toddlers too.
Springfest, Summerfest, Fallfest, Winterfest - Celebrate the first day of each season with appropriate activities.
Age of Aquarius - Celebrate the water bearer zodiac sign with water activities such as; swimming, diving, fishing, water balloon fights, water slides, and hot tubbing. Have bar drinks, but only water! Encourage Sims to indulge themselves in their love of getting water to drink from sinks. Basically anything and everything to do with water! This day will be celebrated on my own birthday, February 5th! :)
Boho/Hippie Day - Dress up as Hippies and go barefoot, grow your hair long and straight, men will grow beards, decorate homes in a Boho/Hippie theme, have a day of free love for all, if I had the bubble blower, they'd use that, dance around a firepit and sit on those puffball seats, play the guitar and tell stories, give and get massages, do yoga and meditate, give a flower to a friend, no fighting, etc., all day will be dedicated to peace, love and acceptance. Namaste!
That's all the custom ones that come to mind at the moment, probably think of others later. Well, Earth Day isn't really custom, it's a real holiday such as the other ones I'll have in my game like...
Thanksgiving, Christmas, New Years, Halloween, Valentine's, Easter, April Fool's, St. Patrick's, Labor Day, etc. I'd love to have some kind of Veteran's or Memorial Day, but we don't have any kind of military in the game right now. Maybe in the future, hopefully! :)