In Ordinary Time, it’s a matter of counting. Some people prefer “ordinal” or counted time to “ordinary.” The story of salvation is never an ordinary event. But we need some way to make the Nativity cycle and the Ninety Days distinct.
103. Ordinary Time begins on the Monday after the Sunday following 6 January; it lasts until the Tuesday before Lent inclusive. It begins again on the Monday after Pentecost Sunday and finishes before evening prayer I of the first Sunday of Advent.
The Order of Readings provides readings for thirty-four Sundays and the weeks following them. In some years, however, there are only thirty-three weeks of Ordinary Time. Further, some Sundays either belong to another season (the Sunday on which the Feast of the Baptism of the Lord falls and Pentecost Sunday) or else are impeded by a solemnity that coincides with Sunday (e.g. The Most Holy Trinity or Christ the King).
It might be accurate to say that every five or six years, the Roman liturgical calendar will have fifty-three weeks. It’s never an even 365-plus days–it’s either 364 or 371. We usually miss a week of ordinary time on the short years. We always miss a Wednesday through Saturday chunk because of Lent. Here’s how it’s organized:
104. For the correct arrangement in the use of the readings for Ordinary Time, the following are to be respected.
1. The Sunday on which the Feast of the Baptism of the Lord falls replaces the first Sunday in Ordinary Time. Therefore the readings of the First Week of Ordinary Time begin on the Monday after the Sunday following 6 January. When the Feast of the Baptism of the Lord is celebrated on Monday because the Epiphany has been celebrated on the Sunday, the readings of the First Week begin on Tuesday.
2. The Sunday following the Feast of the Baptism of the Lord is the Second Sunday of Ordinary Time. The remaining Sundays are numbered consecutively up to the Sunday preceding the beginning of Lent. The readings for the week in which Ash Wednesday falls are interrupted after the Tuesday readings.
3. For the resumption of the readings of Ordinary Time after Pentecost Sunday:
-when there are thirty-four Sundays in Ordinary Time, the week to be used is the one that immediately follows the last week used before Lent; 
-when there are thirty-three Sundays in Ordinary Time, the first week that would have been used after Pentecost is omitted, in order to reserve for the end of the year the eschatological texts that are assigned to the last two weeks. 
Those notes for 115 & 116:
115. So, for example, when there are six weeks before Lent, the seventh week begins on the Monday after Pentecost. The Solemnity of the Most Holy Trinity replaces the Sunday of Ordinary Time.
116. When there are, for example, five weeks before Lent, the Monday after Pentecost begins with the Seventh Week of Ordinary Time and the Sixth Week is omitted.