Зимой я реже гуляю в парке.

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Breakdown of Зимой я реже гуляю в парке.

• парк: the park
• в: in
• зима: the winter

Questions & Answers about Зимой я реже гуляю в парке.

Why is it зимой and not зима or в зиму / в зимой?

Зимой is the normal way to say “in (the) winter” in Russian when talking about a general time period.

• зима = winter (basic dictionary form, nominative)
• зимой = instrumental case, used here in a time-meaning: “in winter / during winter”

Russian often uses the instrumental without a preposition for “in/at (this period of time)”:

• зимой – in winter
• летом – in summer
• осенью – in autumn
• весной – in spring

в зиму / в зимой are not used in this meaning.
You could say зимой or sometimes зимой я…, but you would not add в here.

What case is зимой, and is it always time-related?

Зимой here is the instrumental singular of зима.

Instrumental has several uses (e.g., “with something,” “as something”), but in this sentence it’s used in a temporal way: instrumental of time.

That specific meaning is limited to certain time words (seasons, parts of the day, etc.), for example:

• зимой – in winter
• вечером – in the evening
• утром – in the morning

So: same form (instrumental), but not always about time; here it just happens to be used that way.