Pentecost Sunday: the “Kneeling” Sunday.
Immediately after the end of the Liturgy of Pentecost, the Vespers of the feast of the Holy Spirit is celebrated, which is done with kneeling, because the faithful kneel three times on walnut leaves, when they hear the invitation from the priest “Eti and Eti, bending the knees, of the Lord we pray” and the priest reads aloud wishes to the Holy Spirit.
Worshipers kneel on walnut leaves because they are bitter and symbolize the bitterness that possesses the souls of the dead on this day. According to another tradition, the kneeling is done so that the faithful pray that the bridge that connects the upper world with the lower one does not fall.
Source: 5th Grade Primary School Religion Book