Early Holy Communion

Hope you are all having a blessed Holy Week!

I noticed that the Greek Orthodox churches in my area have an early Holy communion service Holy Thursday and Holy Saturday before each morning Vesperal Divine Liturgy. I am not familiar with this service as the other churches I have attended (Ukrainian Catholic and Antiochian Orthodox) don't seem to offer it. I am just curious as to it's structure and origins. Is it like a Pre-sanctified liturgy?

(Don't worry, I will not be attending or receiving communion :))