Depends on the people involved. I think there would be a big to-do on the anniversary of every major battle, really. Dagorlad, Last Alliance, Azanulbizar, BOTFA, all that. Some people would get megadrunk, cry, laugh, sing and fall over. Some people would get maudlin, or get together with those who were there and reminisce about stuff. IDK, it would be very individual, I expect.
(I think Fili and Kili would spend the whole day with their uncle, trying to lift his mood)