В солнечный день, наоборот, я езжу в парк на велосипеде и долго там катаюсь.

On a sunny day, on the contrary, I go to the park by bicycle and ride there for a long time.

Breakdown of В солнечный день, наоборот, я езжу в парк на велосипеде и долго там катаюсь.

• я: I
• в: to
• парк: the park
• день: the day

Questions & Answers about В солнечный день, наоборот, я езжу в парк на велосипеде и долго там катаюсь.

Why is it в солнечный день and not в солнечном дне?

In time expressions meaning “on / in / during [a day, year, evening]”, Russian normally uses:

• в + Accusative (not the Prepositional).

So you get:

• в солнечный день – on a sunny day
• в этот день – on this day
• в прошлый год – last year
• в понедельник – on Monday

Grammatically, день is masculine; its Accusative singular is день, and the adjective солнечный agrees and becomes солнечный (Accusative masculine inanimate looks like Nominative).

в солнечном дне is grammatically possible but sounds strange here; it would mean something like “in the sunny day” as if the day were a physical space you are inside. For talking about when something happens, Russian prefers в солнечный день with the Accusative.

What exactly does наоборот add here, and is it necessary?