The Day of Pentecost, fifty days after Easter Sunday, observes the birthday of the church when the Holy Spirit came upon the disciples gathered in Jerusalem (Acts 2).
The season after Pentecost is the longest season of the church year and continues until Christ the King Sunday, five Sundays before Christmas. Sundays in "ordinary time" are numbered First Sunday after Pentecost, Second Sunday and so on.
The color for the Day of Pentecost is red. The color used for the Sundays after Pentecost is green. White is the color used on Christ the King Sunday (which is not numbered)which concludes this time in the church's life.
For resources for the Day of Pentecost, visit this link.
For resources for the Sundays after Pentecost, visit this link.