Learn Spanish (Latin America): En fin, mañana será otro día.

En fin is a very common conversational marker used to wrap up a thought, change the subject, or express resignation, similar to 'anyway' or 'oh well' in English. While en definitiva summarizes a point ('in short' or 'ultimately') and al fin y al cabo introduces a bottom-line fact ('at the end of the day'), en fin carries a tone of moving on or giving up, which perfectly sets up the hope for a fresh start tomorrow.

The English translation says 'tomorrow IS another day'. Why does the Spanish use the future tense será ('will be')?

While English uses the present tense for this idiom, Spanish uses the future tense to state an inevitable truth about a time that hasn't happened yet. It emphasizes the progression of time and the promise of a fresh start. Using the present tense here (es) would sound unnatural to native speakers because this is a fixed, recognized proverb.

As a quick refresher, why isn't it un otro día for 'another day'?

It is a very common trap for English speakers to translate 'another' piece-by-piece as un otro. However, the word otro already has the concept of 'a' or 'an' built into it. In Spanish, you never put un or una immediately before otro; it is always just otro día.

Does en fin always have to go at the beginning of the sentence?