In the Collections of build mode in The Sims Mobile there should be an "Events" one. (Besides the current Furnishing, Career and Hobby collections)

Featuring current and past event items as a collection list, it would make it way easier to keep track of what items you've got during the event.
I find it very hard especially during the current Easter Event to keep track of the items I got from the event in my inventory.
Animal Crossing: Pocket Camp also has an "Event" category in the Catalog, with current and past events.