The first time I heard the phrase Hoy por ti; mañana por mí I was amazed at the no-nonsense approach to helping others that it seemed to encapsulate.
The closest we seem to come to it in English is “You scratch my back and I’ll scratch yours”, though I don’t think that’s quite the same, as the English idiom implies a real one-to-one reciprocity.
With hoy por ti I hear a far more general implication of “my turn will come” without it being specified that it’ll be the responsibility of the one who has been helped to do the return favour. Perhaps that’s emphasised by the fact that in the Spanish phrasing, the favour is done with nothing more than a belief in the repayment, whereas in English there’s a demand for a favour first.
I bumped into one of the bank tellers while I was having breakfast today, and when he paid for his coffee he got mine too. Obviously I said it’d be my turn next time I saw him. He coud easily have replied “hoy por ti…”, but instead he said, no problem: hay más días que castaños. Which leaves me with yet another untranslatable – but beautifully seasonal – idiom.