"We are having a party on the thirteenth of September."
Translation:Dyn ni'n cael parti ar y trydydd ar ddeg o Fedi.
this question seems far above the skill level of this exercise and is off topic (family). explain.
This is the effect of the Duolingo algorithm throwing an example at you from a more difficult exercise that I presume you have previously completed while you are re-doing an earlier exercise.