I thought seasonal disorder didnt start until January,personally I am quite upbeat as I have gone full steam ahead into stuff I havent done for the last year and a half (this doesnt include prossies,as I did that) I would only be downbeat if we ever have to go into lockdown again.
Think it's different for everyone, I used to get it years ago sometime in Oct / Nov and sometimes it was like flicking a switch between being happy and a few minutes later being really down, it happened like that if something triggered it.
Used to start feeling better once spring came so usually April / May time but I can recall once still feeling shit in June, perhaps it was just depression as there was a lot of shit going on in my life back then.
It was like I started going into hibernation and was excessively tired all the time, permanently felt like i'd had a couple of pints my head was just fuzzy

I have to say though since I started exercising and going to the gym plus eating plenty of fresh fruit i've never had it since and that is 20 years ago, exercise as they say is a great medicine as it kick starts your body
As a more general though I have thought in the last few years my zest has waned, I no longer wake up at weekends on a bright sunny day in May full of enthusiasm and vigour to get out and about so that i make the most of it while it's good weather, perhaps that's just getting old, you have to start slowing down some time