"They eat with friends on the weekends."
Translation:Ellos comen con amigos los fines de semana.
why not 'en' los fines de semana? "en la mañana" is fine and so is "en la tarde"
No. But it looks like fin de semana simply doesn't require a preposition. This is not completely alien even in English. Consider "we ate on weekend" vs "we ate last weekend". Why adding last makes on unnecessary?
Pretty sure because that limits the sentence to only females, and the sentence doesn't give genders. If there is even one male being talked about, that sentence is wrong.
If we are talking about multiple weekends, shouldn't it be 'fines de las semanas' or something like that? When talking about multiple mondays we say 'los lunes'
In short, no.
When talking about weekends, only the end (fine) part of the phrase is pluralised. It's the same in English where weekends is used, not weeksends.
I am just a learner so if any native speaker can explain this in a better way, I too would be grateful.