"When are you going to Aberystwyth?"

Translation: Pryd dych chi'n mynd i Aberystwyth?

[Mulqueeny]

Why is rwyt ti'n correct? Shouldn't it be wyt ti'n?

[SirBrachis]

This was written by ibisc that you might find helpful:

Questions about how, when, where, why, how often, how quickly, etc, or which start with a preposition or an adverb are generally followed by mae - sut (how), pryd, ble/lle, pam, pa mor aml, pa mor gylfym, am beth, i bwy, gyda phwy, etc.

Am faint o'r gloch...? is asking when, so it is followed by mae.

Other examples:

Sut mae Dewi heddiw? - How is Dewi today?
Pa mor aml mae e'n dod yma? - How often does he come here?
Ble mae e'n eistedd? - Where does he sit?
Gyda phwy mae e'n eistedd? - With whom does he sit?
 phwy mae e'n siarad? - Who is he speaking to? (With whom is he speaking?)
I bwy mae'r anrheg? - Who is the present for? (For whom is the present?)