Trinity Sunday is observed on May 31, 2015. Trinity Sunday is the first Sunday after Pentecost in the Western Christian liturgical calendar, and the Sunday of Pentecost in Eastern Christianity. Trinity Sunday celebrates the Christian dogma of the Trinity, the three Persons of God: the Father, the Son, and the Holy Spirit.
Trinity Sunday is celebrated in all the Western liturgical churches: Roman Catholic, Anglican, Lutheran, (most) Presbyterians, Methodists, and many churches within the Cooperative Baptist Fellowship.
In the Eastern Orthodox and Eastern Catholic churches, the Sunday of Pentecost itself is called Trinity Sunday (the Sunday after Pentecost is "All Saints Sunday"). The Monday after Pentecost is called "Monday of the Holy Spirit", and the next day is called the "Third Day of the Trinity". (With material from: Wikipedia)