Christmas Season begins at sunset on December 24. Christmas Season ends at the Epiphany. Meaning of the Christmas Season is the exchange of divinity and humanity in the incarnation and in human sanctification. Epiphany falls on the 12th day after Christmas - January 6th. Meaning of the Epiphany is establishment of the messianic identity of the child of Bethlehem.
Easter season is fifty days following Easter Day.
Day of the Pentecost. First Sunday after Pentecost is also called Trinity Sunday.