ZaGaikokujin
Weather starts getting bad around mid October (depending on the year the really good weather may last until November) and continues until mid March with lowest numbers btween December-February.
Speaking purely about VFR conditions, lowest what you get in let’s say mid-February is 1-2 days of flying weather followed by 3-5 days of waiting.
Usually there’s also a fortnight of good weather in mid January associated with the Siberian high when the temps drop down to -10 C at day and -20C at night.
From April it gets really good, best is May, then the summer heat and thunderstorm season come (still good weather) and it lasts until October (second best month after April IMHO).