一天比一天 / 一年比一年【yì tiān bǐ yì tiān / yì nián bǐ yì nián】

一天比一天 is a 比 comparison folded back on itself: each day is more so than the one before. The same time word appears on both sides of 比, and the adjective comes last. A final 了 marks the change as already under way.

With a measure word

With 个, 件 or another measure word instead of a time word, it compares the members of a group: each one more so than the last, so all of them very.

Bare adjective

As in any 比 sentence, the adjective takes no 很 or 非常.

• × 天气一天比一天很冷 → 天气一天比一天冷

越来越 also means "more and more", and the two don't stack: pick one.

• × 天气一天比一天越来越冷 → 天气越来越冷 or 天气一天比一天冷

一天比一天 sounds measured, change visible at each step; 越来越 is the general word.