The noun 夏日 (xiàrì) means "summer day" or "summer" and is a slightly literary way to refer to the summer season. It combines 夏 (summer) and 日 (day/sun), evoking warm, sunny imagery. It is more poetic than the everyday term 夏天.
夏日的阳光照在湖面上,特别好看。
Xiàrì de yángguāng zhào zài hú miàn shàng, tèbié hǎokàn.
The summer sunlight shining on the lake looked especially beautiful.
这是一个难忘的夏日午后。
Zhè shì yī gè nánwàng de xiàrì wǔhòu.
It was an unforgettable summer afternoon.
夏日炎炎,大家都躲在空调房里不想出门。
Xiàrì yányán, dàjiā dōu duǒ zài kōngtiáo fáng lǐ bù xiǎng chūmén.
The summer heat was scorching, and everyone hid in air-conditioned rooms, not wanting to go out.