Q: When is ordinary time quite extraordinary? When it is a season of the church. In the seasons after the Epiphany and after the Day of Pentecost, the Sundays are numbered first, second, third, etc. That is, they are anchored to a particular event and counted after that event with ordinal numbers, hence the term "ordinary time". These seasons are marked using the color green, signifying growth.
In the ordinary time after the Epiphany, we focus on the increasing light of God's revelation of Jesus as the promised Savior and Messiah. The scriptures show us, week after week, who Jesus is, how he embodies the mighty power of God, and how his life is indeed light for the world. God's people are gathered into Jesus' presence and sent out into the world to shine His light through their lives.
In the ordinary time after Pentecost, we focus on growing as disciples of Jesus as we hear the scriptures of his teaching, parables, and call to follow him.
Q. Why aren't other seasons ordinary time? A. Because the event that defines them is more than a day. Christmas is celebrated as a season of (at least) twelve days beginning with the birth of Jesus. The Nativity of our Lord is a multi-day event, including the visit of the shepherds, the circumcision, and concluding with the visit of the magi. Easter is celebrated as a "week of weeks", that is, seven weeks ending with Pentecost on the 50th day. The Sundays in these seasons are Sundays of Christmas/Easter.