"Where are you going on the seventeenth of September?"
Translation:Ble dych chi'n mynd ar yr ail ar bymtheg o Fedi?
Either ...o fis Medi or ...o Fedi is fine here - it is clear from the '17th' in the rest of the sentence that it is about a date, so the mis can be omitted without any danger of misunderstanding.
(The other potential translation of the sentence if spoken would be the nonsensical '...the 17th of harvesting'.)
As a learner, though, it is probably better to stick with mis Medi so that you get into the habit of using the mis.