"Táim ag snámh ar an Satharn."
Translation:I am swimming on Saturday.
ar an Satharn is just the phrase to express "on Saturdays". Really, I think this sentence is wrong, as ar an expresses habitability (See FGB entry where "Ar an Satharn" means "On Saturdays"). So, really, it should be Bíonn mé (or bím) ag snámh ar an Satharn or Tá mé ag snámh Dé Satharn.