"what is liturgical calendar for?"
Basically, to keep track of events in the liturgical year, because the Orthodox liturgy, as it is practiced over the course of a year, is extremely complex. In fact, the word 'complex' doesn't really do it justice.
The yearly liturgy consists of cycles of fixed feasts, moving feasts, saint's remembrances, commemorations of events in the life of Christ, in the life of the Theotokos, fasting periods, and other occasions too numerous to cite. The year is sub-divided into seasons, the seasons into months, the months into weeks, the weeks into days and the days into hours. The number of liturgical books that must be consulted in order to perfom a day's liturgy almost boggle the mind.
Look up the word, 'byzantine'.