Birthdays are typically days of great joy, both for the person celebrating it and for their family and friends.
Why shouldn’t the birthday of St. John the Baptist similarly be a day of joy?
Here is a short prayer from the Raccolta that asks God for the grace of spiritual joy.
O God, Who hast made this day to be honorable in our eyes by the Nativity of blessed John the Baptist, grant unto your people the grace of spiritual joy, and direct the minds of all your faithful into the way of everlasting salvation. Through Christ our Lord. Amen.