I heard somewhere that you must be at Divine Liturgy before the reading in order to receive communion. I expect that some priests might say that you should arrive prior to the start of Liturgy and perhaps even orthros. Me and my family are always early–probably because of my Roman Catholic upbringing.
There is no “obligation” in the Orthodox Church. Sunday Liturgy is seen as a “privilege” to attend. And we do not have the concept of the “Saturday eve is the same as Sunday”.
Saturday eve is for vespers.