September.
First month where I start noticing the dark drawing in in the evenings, it gets harder to get out of bed every morning. "Tis no ease to rise on a grey day. The devil holds fast your eyelids."
The weather seems to get very gloomy and wet very quickly, but it's not quite cold enough yet to crack out all my lovely winter clothes so I'm stuck in a weird sweaty humid limbo until the proper crisp autumn weather sets in.
All the bugs in the universe are both horny and dying and want to shag and die in my living room.
All the parents off over the summer hols come back to work and September is the quietest month anyway, so suddenly the work load drops off and I find myself bored and twiddling my thumbs for hours (I played minesweeper for 4 hours on Friday, first world problems I realise).
Said parents are always cranky for the first few weeks while getting back into the school routine, and the general mood in work is just low and grumpy after a couple of months of hectic energy and high spirits.
It's too early to get excited about Christmas and I don't care about Halloween so it just feels loooooong.
I just find it the most blah month of the year.