You'll Wish
 

If I don't do it now, tomorrow you'll wish I had done it.

Si no lo hago ahora, mañana desearás que lo hubiera hecho
Si no lo hago ahora, mañana desearás que lo haya hecho

Since "you'll wish" (desearás) is used, does what follows need to be in the subjunctive? And if so, does it have to be "haya" hecho or is "hubiera" hecho permissible?

The situation today (to do what is necessary) will be a past thing tomorrow, so the necessary conjugation is "hubiera hecho".

"Haya hecho" would mean the other person can still hope tomorrow that you had done the thing before they regret not having let you do it.
- Rezo para que José no haya hablado con Pedro antes de hacerlo yo. -> When I talk to Pedro, I'll be hoping José hasn't talked to him yet.
- En la noche desearás que haya llovido para no tener que regar tu jardín. -> If it rains today, you won't have to hose your garden tonight.
- Al final del año desearemos que se hayan terminado todos nuestros problemas. -> By the end of the year we'll wish all our problems will be over.

Just out of curiosity, another way of saying this is: If I don't do it today, I will regret it tomorrow. I fear that my translation of this without the subjunctive is incorrect.
This is my translation: Si no lo hago hoy, mañana tendré remordamientos/ o mañana lo lamento.
Is this correct?

Thank you, AlgelicaDeAlquezar. Your explanations make it clear. I appreciate your taking the time to write them.

@Poli: The subjunctive is triggered by the verb "desear", so your proposal: "si no lo hago hoy, mañana lo lamentaré/tendré remordimiento(s)" is correct, as both verbs correspond to the moment:
- "hago" (present -> "today")
- "lamentaré"/"tendré" (future -> "tomorrow")


You might hear "mañana lo lamento", but this is very colloquial speech. Not recommended. ;)

@Bobbert: Glad to help. :)