There is nothing “ordinary” about “Ordinary Time”. Ordinary Time is not about common, regular, mundane, or run of the mill. Ordinary Time comes from the word “ordinal” as in “ordinal numbers”. Remember your Maths: Cardinal numbers answer “how many?” “Ordinal Numbers” tell the rank, they answer “what position?” Ordinal Numbers are first, second, third, fourth, etc. There are normally fifty-two weeks in a year. These are made up of the Advent, Christmas, Lent, and Easter Seasons. Normally that leaves thirty-four weeks of “Ordinary Time”. Those weeks start from The Baptism of the Lord up to Lent, and start again at the Day of Pentecost.
The Day of Pentecost ends the fifty day season of Easter (that’s what the Greek word “Pentecost” means!) It does not begin a “Pentecost Season”. In the Nicene canons we are forbidden to kneel on Sundays and the Bishops at the Council of Nicaea were horrified to hear of people kneeling during Pentecost – by which they meant the fifty days of what we now call the Easter Season (Council of Nicaea, Canon 20). During the week following the Day of Pentecost, the collect is that of the sixth week in OrdinaryTime. During the week following Trinity Sunday the collect is that of the seventh week in Ordinary Time. Trinity Sunday also is a feast, not the start of a season.