Repairing the game / deleting cache files didn't help but it seems like it is a smaller bug than originally thought.
Can add holidays to the three seasons that the Sim is currently not in as well as the current season. However can't add holidays to the season that the Sim is currently in when it pops up at the end of the calendar. For example if my Sims are in Spring - then Spring weeks will appear at the end of the calendar for the next season and can't add holidays to this. However once Sims enter into a new season (summer) then the holidays re-appear in the Spring.
Sorry if this is confusing but it is the best way I can think to describe it.
Comments
Repairing the game may help. To do that, go to games library in Origin, right click on the sims 4 picture and click repair
Try deleting the cache files and see if that helps
https://sims4.crinrict.com/en/2014/09/delete-cache-files/
If the issue still remains, try starting another fresh new save game file and see if the issue occurs there also.
http://forums.thesims.com/en_US/discussion/817478/hello-come-and-introduce-yourself
http://tinyurl.com/OneRoomOneWeek
http://tinyurl.com/rosemow
My Showcase thread https://forums.thesims.com/en_US/discussion/948861/rosemow-s-rooms-showcase
Can add holidays to the three seasons that the Sim is currently not in as well as the current season. However can't add holidays to the season that the Sim is currently in when it pops up at the end of the calendar. For example if my Sims are in Spring - then Spring weeks will appear at the end of the calendar for the next season and can't add holidays to this. However once Sims enter into a new season (summer) then the holidays re-appear in the Spring.
Sorry if this is confusing but it is the best way I can think to describe it.
http://forums.thesims.com/en_US/discussion/817478/hello-come-and-introduce-yourself
http://tinyurl.com/OneRoomOneWeek
http://tinyurl.com/rosemow
My Showcase thread https://forums.thesims.com/en_US/discussion/948861/rosemow-s-rooms-showcase