I think I do have my Valentines day in spring.
I have Christmas about the third week of Winter For Christmas season I have a holiday the day before Christmas Eve for baking holiday treats, my mom always made a goodie box with fudge and cookies for everyone. So, I kind of continue that by baking holiday cookies, brownies etc. and putting up decorations with my sims, and now that we have group cooking, it works out perfectly for families. then I have Christmas Eve, where I invite over family members and give each person a gift, and have dinner. On Christmas, sims open the presents around the tree on Christmas morning, then again invite relatives over for a big feast and reminiscing. Christmas is always celebrated with families with children. I have aging turned off so different years different sims ( in my rotation ) celebrate it.
New Years Eve and New Year’s Day are at the end of winter, often celebrated with couples. Drinks and New Years resolutions, often out with friends somewhere. I will have to add the new champagne bottle and toast from the new pack to that.
Valentine’s Day I will usually have my sims make a vase of flowers for their Significant other ( wish we had candy making in the Sims 4 ), then they will go out for a night on the town, sometimes with other couples, or sit at home and watch a movie together. Early/mid Soring.
I have Easter with various activities, usually a big feast and egg hunts. Another event celebrated with families with children usually, in late Spring
4th of July and a BBQ with fire works if sims have purchased them, usually with families with children, a summer holiday.
During the year various sims will take vacations.
I have scout clubs, but am going to add a vacation where I take child/teen sims that are scouts and put them in one household for a 3 day summer vacation
My archeologist and writer sims take sabbaticals.
I think I have a Memorial/Labor Day holiday too.
My sims have a Halloween holiday in late summer or early fall. There will usually be a costume party, so it’s usually at sims that like to entertain.
And, I have Thanksgiving too, usually an extended family event in late fall, with a big dinner, drinks and lots of chat,
I almost always have sims take off the day during holidays, and I use the 4 week season.