Seasons Mods - Sing and Wear Costumes Always Enabled
With this mod, Sims will always be able to do the group sing and wear costume interactions. Sims will also always be able to sing together by clicking on fireworks, cakes and present piles.
By default, these interactions are only available during an active holiday.
If Sims do the group sing interaction nearby a Sim who recently aged up due to their birthday, that Sim will clap and happily cry as a reaction to the other Sims singing. This extra feature will only show if you have Lovestruck installed, though, as it uses animations that are probably restricted to that pack (not 100 % sure about that, so any feedback whether it's also working without Lovestruck installed is welcome).

Required Packs: Seasons
Compatiblity: Will conflict with any mod that alters these tuning files (except for my own as I made them compatible):
trait_Species_Human, 151039
buff_HolidayTraditions_Hidden_Sing_GrandMeal, 189826
groupSing_LeaderSingInteraction_HappyBirthday, 188138
Note: EA clothing that has been assigned for use with this mod will be seen as CC by the game. If you want to create a Sim to share in the gallery, temporarily delete the mod. You can also save already existing Sims anew after deleting the mod. They will then show as CC-free.
Creator website: https://www.seebee.de/sims4me/