暑い日だった

was a hot day

→ 暑い
phrase Rare #34,124
was a hot day 暑い describes hot weather or temperature. The plain past form だった makes the phrase casual.
JA: 先週は暑い日だった。
EN: Last week was a hot day.

とても暑い日だったので僕たちは泳ぎに行った。

It was a very hot day, so we went swimming.

大変、暑い日だった。

It was a very hot day.

昨日は暑い日だった?

Was it hot yesterday?

とても暑い日だった。

It was a very hot day.