I would say commemorate is the nearest we would have. You might also use 'mark'

You can use 'commemorate' for a memorial day, but it wouldn't be a positive way to describe a more joyful occasion.

The opposite of "celebrate" is "mourn". However, we would only use the verb "mourn" for a funeral, when somebody has died. For remembrance days, the verb we use is "observe". (I had to leave this as a comment, rather than as an "answer", because your question does not appear to be open to new answers).