Metro is completely correct in that there is no way to tell.
However, there have been a number of reports on here that Sunday tends to be the least crowded day of the week. These are anecdotal, so I don't know how much stock you want to put on that.
So I would say that mid-week (Tue-Thu) or Sunday, in that order, would be your best choices. Stay away from Fridays and Saturdays in particular at any time of year.