天

天 already handles the counting job for days, so a number in front is all you need. Saying 一个天 is like hiring a second counter for a job that's already done. The same logic runs through the whole sentence: 两三年, 四五天, 五分钟 — no 个 anywhere. Once you see 天 as the counter and not just a thing being counted, the mistake stops feeling tempting. “工作两三年，休息四五天，读书一小时，吃饭五分钟。” (Work for two or three years, rest for four or five days, read for an hour, eat for five minutes.) ── [《一二三（2）》](/read/counting-days-rhymes#s6)

每天 is the neutral, safe pick for writing, work emails, and news. 天天 carries the same meaning but feels warmer or mildly annoyed, like a small eye-roll. If you're complaining that a coworker is late every single day, 天天 lands harder. Both are correct, so choose by tone, not by grammar rules.

Standing alone, 天 usually points up at the sky: 天很蓝, 天上有一只鸟. Once you glue it to a time word like 今, 明, 昨, or 每, it flips into 'day'. The character never changed — the word sitting next to it tells you which meaning is live. Reading the neighbor, not the character, is the trick.