The liturgical calendar“maps out", if you will, events that we remember about the Paschal Mystery, as well as events and saints that remind us about God’s mercy. It revolves around a week that peaks when we remember and make-present the sacrifice of Christ every Sunday.
The weeks can encompass larger units of time we call seasons like Advent when we long for the coming of Christ, Christmas when we are overjoyed in the nativity of Christ, Lent when we commemorate the Passion of Christ, and Easter when we rejoice in the Resurrection of Christ.
There are some weeks, however, that don’t fall in any of the seasons, so we“count" them instead. That is why we hear First Sunday, or Second Sunday, or Third Sunday and so on. A number that defines a position in a series is called an ordinal number, or ordinal for short. This is why we call time that doesn’t fall in a season,“ordinary time" because it is counted. In this sense,“ordinary" doesn’t mean not special, or common.