Today's celebration is the midpoint of the fifty days between the Feasts of Pascha and Pentecost. St John tells us (John 7:14) that "in the midst of the feast Jesus went up into the Temple, and taught." The Feast in question is the Feast of Tabernacles (celebrated in September), not Pentecost.
The Church has appointed John 7:14-30 to be read for the Midfeast, thereby linking Pascha and Pentecost. In Chapter 8 of St John's Gospel, the Lord came to the Temple again and taught the people who came to Him. After leaving the Temple, he encounters the man born blind. We will hear about him on the Sunday of the Blind Man.
The Troparion of the Midfeast ("In the middle of the Feast, O Savior, fill my thirsting soul with the waters of godliness, as Thou didst cry to all: If anyone thirst, let him come to Me and drink [John 7:37]. O Christ God, Fountain of our life, glory to Thee!") hints at the encounter of Christ and the Samaritan Woman in just a few days.
Today we perform the Lesser Blessing of Water, and the Blessing of Fields.
Text taken from the OCA Website
Troparion - Tone 8
In the middle of the Feast, O Savior,
Kontakion - Tone 4
Christ God, the Creator and Master of all