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.

The Sundays following Pentecost, until Advent, are numbered from this day. In traditional Roman Catholic usage, the First Sunday After Pentecost is on the same day as Trinity Sunday. In the revised Roman rite, Ordinary Time resumes one week earlier, on the Monday after Pentecost, with the Sundays that would otherwise fall on Pentecost and Trinity Sunday omitted that year. In the Church of England, following the pre-Reformation Sarum use, the following Sunday is the "First Sunday after Trinity", while the Episcopal Church in the United States of America (ECUSA) now follows the Catholic usage, calling it the Second Sunday after Pentecost. The liturgical colour used on Trinity Sunday is white.

Sunday (ⁱ/ˈsʌndeɪ/ or /ˈsʌndi/) is the day of the week between Saturday and Monday. For most Christians, Sunday is observed as a day for worship of God and rest, due to the belief that it is Lord's Day, the day of Christ's resurrection.

Sunday is a day of rest in most Western countries, part of 'the weekend'. In most Muslim countries, and Israel, Sunday is a working day.

According to the Hebrew calendars, traditional Christian calendars, Sunday is literally the "first day" of the week. According to the International Organization for Standardization ISO 8601 Sunday is the seventh and last day of the week.

No century in the Gregorian calendar starts on a Sunday, whether its first year is '00 or '01. The Jewish New Year never falls on a Sunday. (The rules of the Hebrew calendar are designed such that the first day of Rosh Hashanah will never occur on the first, fourth, or sixth day of the Jewish week; i.e., Sunday, Wednesday, or Friday).

The English noun Sunday derived sometime before 1250 from sunedai, which itself developed from Old English (before 700) Sunnandæg (literally meaning "sun's day"), which is cognate to other Germanic languages, including Old Frisian sunnandei, Old Saxon sunnundag, Middle Dutch sonnendach (modern Dutch zondag), Old High German sunnun tag (modern German Sonntag), and Old Norse sunnudagr (Danish and Norwegian søndag, Icelandic sunnudagur and Swedish söndag). The Germanic term is a Germanic interpretation of Latin dies solis ("day of the sun"), which is a translation of the Ancient Greek heméra helíou. The p-Celtic Welsh language also translates the Latin "day of the sun" as dydd Sul.