The Christian Church was borne out of the mixed soil of the Old Testament Scriptures, first-century Judaism and the Græco-Roman world. This mixture of influences is still with us today when it comes to measuring and marking time in the Church’s life.
The worship of the Old Testament, and much of the Judaism of Palestine in the first century, operated with a lunar calendar, where the change of months was determined by the cycle of the moon. Months, and therefore festivals, would not always occur at the same time of the natural year.
This is why the chief festival of the Church, Easter, which is based on the Jewish festival of Passover, can occur on any time between 22 March and 25 April, depending on when the first full moon of the spring occurs.
On the other hand, the Romans (like us) used the solar calendar, so that months always occur at exactly the same time of the natural year. Festivals that have no Old Testament precedent but were introduced by Christians, such as Christmas, therefore have a fixed date.
Some of the seasons of the Church Year relate to fixed dates (e.g. Advent for the 4 Sundays before Christmas), some to movable dates (e.g. Lent for 40 days before Easter), and others vary depending on how the two relate (Epiphany and Pentecost).
The Church Year has kept both calendars side by side, giving us a number of fixed festivals, with Easter moving to and fro. As a result, in any given year, we might have a short Epiphany season and a long Pentecost (Trinity season), or vice versa.