Since you seem to mean a locked timeline where we can’t change what we did the last time (?), any day involving lots of fun and/or love. I’ve had many of those.
Without a doubt the birth of my first child, my son Ryan.
That day was priceless & one I would gladly repeat over & over.
Was the same when my daughter was born, but becoming a father for the first time just blows your socks off!