Learn French: Le lendemain de la fête, nous étions fatigués.

Demain means tomorrow relative to right now. When narrating a past event, the next day is always le lendemain. Also, avoid le prochain jour; in French, prochain is used for future dates relative to today (like mardi prochain), not for sequencing events in a past-tense story.

Why is it le lendemain de la fête and not le lendemain après la fête?

The noun le lendemain already implies the day after. To connect it to an event, you simply use the preposition de (meaning of). Using après would be redundant, as if you were saying the day after after the party.

Why use the imperfect étions here rather than the passé composé avons été?

The imperfect tense is used here because being tired is an ongoing physical state or background condition in the past. If you used the passé composé (nous avons été), it would sound like a sudden, completed action rather than a continuous feeling that morning.

Does fatigués change if the speakers are all women?