If you're talking witches, sooner or later you gotta talk Sabats. I had to make some concesssions and adaptations to incorporate them into Wytch Mountain, but overall I think it works. For the sake of this game, I only included 6 because we use a d6, not a d8 and I wanted it to be randomized. With 5 classes, each Sabbat gives advantage to one of the classes. (Imbolc serves as a neutral time where no witch has a special advantage.) 

Before beginning play, you roll a 1d6 to see what the next Sabat is and a 1d20 to see how many days away it is. 

1 Beltane When nature is rejuvenating and blooming, a witch's power is closely tied to growth, fertility, and transformation. When instructed to roll 1d6, Hedge Witches may choose their result.

2 Litha When the sun is at its zenith, a witch's power is at its peak. Their spells focus on harnessing solar energy, enhancing vitality, and working with fire-related magic. Warlock blades do an additional 1d6 points of damage for each level the Warlock is at.

3 Mabon As leaves change and nature prepares for the cold months, spells involve divination, ancestral magic, and working with the energies of change. The thinning veil makes it a good time for communing with spirits and seeking guidance from the beyond. Black Mass Witches cannot have their Tainted Grail destroyed.

4 Samhain When the veil between worlds is at its thinnest, the dead walk close and old bargains come due. Witches draw power from endings, memory, and the hunger of things that should not still be moving. All witches may cross into the Otherworld (Leyline Witches  require no SP), but the Hexkeeper rolls on the Otherworld Encounter table twice and both results are present simultaneously.

5 Yule On the longest night, a witch's magic turns toward endurance, hidden warmth, and the slow return of things thought lost. Curses cast during Yule cannot be broken until the following Beltane. Potion Witches regain all SP at midnight rather than after a full rest.

6 Imbolc During the cold months, a witch's power turns inward. Their magic focuses on protection, warding, and drawing upon the energies of the earth and the stillness of the season. Winter is also time for deep meditation, astral travel, and accessing hidden knowledge. No witch gets special powers on these long dark nights.