Several parts combine into one character.

Oracle bone 夏 pictured a person with head, hands, and feet all spread out — the largest, most expansive human pose. The summer-meaning emerged from this "biggest, most active" association. Identical across 繁體 / 新字体 / 简体.

A fascinating cultural note: 夏 was also the name of the legendary first Chinese dynasty (Xia, c. 2070–1600 BCE), and "華夏 / 华夏" (Huáxià) became one of the classical self-designations for the Chinese people themselves. So this character carries the weight of both "the active season" and "us — the ancient civilization". The English term "Sino-" derives indirectly through this etymological route via 秦 (Qin), but 華夏 remains the dignified internal name.

Mandarin: xià, falling 4th tone. 夏天 (xiàtiān, summer), 夏季 (xiàjì, summer season), 夏令时 (xiàlìngshí, daylight saving time), 立夏 (lìxià, "establishment of summer" — the East Asian solar calendar marker around May 5). The dynasty name is also 夏朝 (Xià Cháo).

Japanese: TWO on-readings. カ (ka) is more common — 夏期 (kaki, summer period), 初夏 (shoka, early summer). ゲ (ge) appears in 夏至 (geshi, summer solstice — June 21). Kun-reading なつ (natsu) is the everyday word — 夏 (natsu), 夏休み (natsuyasumi, summer vacation — every Japanese student's favorite phrase), 真夏 (manatsu, midsummer), 夏祭り (natsumatsuri, summer festival), 夏服 (natsufuku, summer clothes / school uniforms).

The Japanese 夏祭り (summer festivals) — yukata-clad street festivals with paper lanterns, taiko drums, and shaved ice — anchor the entire summer cultural calendar in Japan, dramatized in countless anime and movies.

Memory aid: a person stretched wide — the season of full bloom.