"The Church" in my homebrew is a polytheistic variation on the powerful Catholic church of the middle ages, so I take much inspiration from there.
Blessings of the Fleets and Fields - in spring, there are a number of rituals as the village priests invoke the goddess of the harvest and bless the fields. Around the same time, the priests in coastal villages also invoke the blessings of the goddess of the seas, on behalf of the fishermen.
Gathering of the Knights - the big knightly order has a gathering in each of the Spring, Summer, and Autumn seasons. Winter is just too cold to move around that much.

The summer one is held at their main stronghold. This is a big occasion for tournaments, and it has become local tradition for there to also be a festival held. The other two are held at other strongholds, in rotation, and are occasions for lesser festivals and tournaments there.
Midsummer - the celebration of the height of the growing season. This one is very big for the Druidic orders, and for the devotees of the harvest/grain goddess.
Midwinter - Appeasing the reaper, this one is used to ward off the attentions of famine, plague, pestilence, and to acknowledge the passage of the dead. Common to both the Church and the Druids.
MidSpring - Planting season, usually when the blessings of the fields and fleets occurs.
MidAutumn - Harvest, another celebration, giving thanks for the bounty as it is stored away for the winter.
Other odds and ends:
The Day of Locking - the anniversary of when the Lich King was imprisoned. Celebrated by locking, and keeping locked, containers and chests. It is considered unlucky to have to open a locked object on this day.
The Rebuke - At one point, the priests serving the god of magic had been hoarding the spellbooks of dead mages. The god of magic took exception to this, and struck their temple, scattering the pages of spellbooks to the winds. It is still possible to find, in odd places, a few pages of a spellbook with a complete spell on them.
Having multiple moons (4) makes for some interesting phenomena:
Night of Doom - Once every great while, all 4 moons are dark on the same night. Such a night is greatly feared, as it represents a time when the forces of Darkness are at their greatest strength.
Night of Light - Midway between occurrences of the Night of Doom, the moons are all full together. This marks the time when the powers of Light are their full strength. Such a rare day is highly celebrated, and marked with great festivals.