April 21 -22
Held on the sixteenth day of the month of Mounichion, the festival in honor of Artemis Mounichia is celebrated the Hellenic defeat of the Persians at the battle of Salamis. Cakes adorned all around with candles were offered to the goddess Artemis and young girls were dressed as bears as in the Brauronia.
The battle of Salamis was fought in September of 480 BC in the straights between Salamis Island and Piraeus near Athens.
This Wikipedia article is useful to get an idea of the battle.
I have chosen to offer up some cakes to Artemis in remembrance of this day.