I haven’t been there but many relatives and friends have been! It’s a true blessing to venerate the icon of Our Lady of the Snakes (Fidousa) and to touch these snakes, that are totally harmless!
According to one theory, when the pirates docked at Katelios and reached the monastery, the nuns ran to the bell tower as the convent burned in fear of being raped and captured for slavery. There they prayed to be transformed into snakes so as to escape unharmed. God answered their prayer and they slithered away to safety. Others say the nuns merely prayed for protection while the convent was burning, and suddenly snakes appeared everywhere around the convent, frightening the pirates away. What can be said for sure is that according to the tradition, snakes played a significant role in somehow saving the nuns, and since then they have appeared every year during the feast of the Dormition.
I hope this helps! God bless you!