Хотя день и был длинным, вечером мне стало спокойнее, когда я полил кактус и выключил телефон.

Although the day was long, in the evening I felt calmer when I watered the cactus and turned off the phone.

Breakdown of Хотя день и был длинным, вечером мне стало спокойнее, когда я полил кактус и выключил телефон.

• я: I
• день: the day
• быть: to be
• телефон: the phone

Questions & Answers about Хотя день и был длинным, вечером мне стало спокойнее, когда я полил кактус и выключил телефон.

What is the job of и in Хотя день и был длинным?

This и is a particle that often appears in the pattern хотя ... и ....

It does not usually need a separate translation into English. It adds a concessive feel, something like:

• although the day was long
• even though the day was long

So:

• Хотя день был длинным = correct
• Хотя день и был длинным = also correct, and a bit more idiomatic or emphatic in this kind of sentence

English speakers often expect и to mean only and, but here it does not mean and.

Why is длинным in the instrumental case?

Because after быть in the past tense, a predicate adjective in its full form can appear in the instrumental.

So:

• день был длинным = the day was long

This is a normal pattern in Russian. You may also see nominative in some cases:

• день был длинный

Both are possible, but the instrumental often sounds more like a description or characterization of the situation/state. In this sentence, длинным is natural and standard.

Why is вечером used without a preposition?