Tough choice, but I'm going with Saturday.
I get to sleep as long as I want, still knowing that Sunday can be turned into a productive day if I need to and there's just something about the way the word looks and sounds that really appeals to me. It's just a day with a lot of potential. You can be lazy or go to a party or stay in bed under the covers with a lover all day. It's just the only day of the week that I never associated with work or adult responsibility of any kind.
Say "just" much? LOL Sorry about that.