On the Sunday before the Feast of Great and Holy Pascha and at the beginning of Holy Week, the Orthodox Church celebrates one of its most joyous feasts of the year. Palm Sunday is the commemoration of the Entrance of our Lord into Jerusalem following His glorious miracle of raising Lazarus from the dead. Having anticipated His arrival and having heard of the miracle, the people when out to meet the Lord and welcomed Him with displays of honor and shouts of praise. On this day, we receive and worship Christ in this same manner, acknowledging Him as our King and Lord.
Source: Church Calender