Quick answer
“In winter” and friends use the bare instrumental: зимо́й, весно́й, ле́том, о́сенью. No preposition at all.
The seasons
зима́zimáwinter
весна́vesnáspring
ле́тоlétosummer
о́сеньósenautumn (f.)
вре́мя го́даvrémya gódaseason
времена́ го́даvremená gódathe seasons
In that season
зимо́йzimóyin winter
весно́йvesnóyin spring
ле́томlétomin summer
о́сеньюósenyuin autumn
кру́глый годkrúglyy gotall year round
What happens
та́етtáyetis melting
цветётtsvetyótis in bloom
жара́zharáheat
листопа́дlistopátfalling leaves
моро́зmorózfrost
пе́рвый снегpérvyy snekthe first snow
Talking about the year
Зимо́й хо́лодно.It’s cold in winter.
Ле́том мы е́дем на мо́ре.In summer we go to the sea.
Весно́й всё цветёт.In spring everything blossoms.
О́сенью ча́сто дождь.In autumn it often rains.
Како́е вре́мя го́да ты лю́бишь?Which season do you like?
О́сень is feminine
Three seasons behave as you would expect; о́сень ends in -ь and is feminine, which gives the unusual instrumental о́сенью with no ending vowel. Compare но́чью — at night, from the same noun type.
The Russian calendar year
Winter in Russian terms is December to February, and it is long: snow often lies from November to March. Весна́ means slush as much as blossom, and the brief ба́бье ле́то — “women’s summer”, our Indian summer — is the warm spell in September that everyone looks forward to.
Typical mistakes
в зиме́
зимо́й
Seasons take the bare instrumental.
в о́сени
о́сенью
Same rule, feminine -ь form.
Check yourself
How do you say “in autumn”?
The bare instrumental.