Completely Depends on business strategy.
Generally the best time (for MS/Sony) is towards the end of the year such as September, October or November. I would generally say October as pre-orders will keep that month busy just before the sales shoot up in Nov/December. So the console will have 3 healthy months instead of just 2.
...But
if it takes a while for a good line-up of 1st/3rd party games (particular 3rd party in Nintendo's case), then early on in the year (but not January or February) to let game releases build up towards Christmas so that player can choose between a good selection of games (like the Switch did).
I have (or have/had in the household): ZX Spectrum, Commodore 64, Amiga, NES, Sega Master System, Super Nintendo, Sega Megadrive, Gameboy, Playstation, Nintendo 64, Windows 95, Gameboy Colour, Windows 98, Sega Dreamcast, Gameboy Advance, PS2, Gamecube, Xbox, Windows XP, Nintendo DS, Xbox 360, Wii, PS3, Windows Vista, iPhone, Windows 7, 3DS, Wii U, PS4, Windows 10, PSVR, Switch, PS5 & PSVR2. :D
and I Don't have: Magnovox Odyssey, Any Atari's, Any Macintosh computers, Sega Gamegear, Virtual Boy, Sega Saturn, N-gage, Xbox One, Xbox Series X/S, PSP, PSVita & Andoid Phone. Plus any non-main-stream consoles/platforms I haven't mentioned.
