Well, I probably wouldn't use this, but if Coppermine had online calendar, user could click on dates to see if there are pictures for that particular month/date.
Some friends of my asked for a calendar too, but not exactly like what you asked.
It'd be good if you can make a calendar where all of the days are actually albums.
Lemme clarify a bit:
Let's say you're on holiday and you make pics everyday, you'd want to upload the pics of one day to one album.
If there is a calendar, they can click on it to get the pics of that day of your holiday.