I'd have to say, "Monday." Although I like the idea of new "beginnings," and Monday can signify the start of a brand new week, it brings it's own "rain," so to speak.
On the personal side:
Our well-intentioned plans are often dashed, or unforeseen issues pop up that we might interpret as "obstacles and interruptions," making us feel pressured in terms of lost time spent on our intentions. Bottom line being, we think, "Off to a bad start!"
On the Business side:
Bright and early Monday morning, the calls begin to come in non-stop, from current clients, new clients, and business associates. It's the day to tie up the loose ends from the previous week while attempting to create the new week agenda. Just a lot of spinning in circles, throughout the day, until you're firmly back into the pace of the old grindstone.
In my particular case, Monday would be the day I dislike the most. However, I am always working on changing negatives into positives. I'm hopeful I'll get these Mondays under control!! Which is your least favorite day, Sunshine??