I rather think that the Faith of the Seven is closer to real world pagan beliefs than Catholicism, but it *feels* that way because of how it is interpreted, with Septons and Septas; effectively monks and nuns, with the hierarchy placing the men first. If you look at the archetypes, you have Maiden/Mother/Crone (triple aspect female) and the Warrior/Father/Smith (triple aspect male) and the Stranger (death, the great leveller). It has the potential to be a very balanced religion, especially with regard to male and female equality, as with most pantheistic religions. Modern paganism reflects this. However, my suggestion is that in GoT, the Faith of the Seven suffers from how humans (chiefly the Septons) have interpreted their gods, which subjugates women in this quasi-medieval world. It also develops fanatics, who are prepared to resort to violence to enforce their beliefs. We've seen that before, many times.
As to holidays, I'd sugest you could follow the old European pagan calendar with the seasons being heavily involved. Plus, in ancient days, being Pagan simply meant you lived in the countryside and had different beliefs from folk in the cities/towns. People who live rurally, who work in the fields and forests tend to have more of a handle on the turning seasons, weather patterns, and the way animals live; breeding seasons, migrations, etc. City dwellers are more in tune with making, availability of materials, trading seasons, weather (eg: firing pots in winter isn't easy, antlers are shed only in spring, traders might only arrive spring and summer). Different considerations might drive your beliefs, which god you invoke and when.
I haven't even touched on the Old Gods, because I feel they are rather more pagan in Westeros than the Seven. However, merely the aspects of the Seven give me more pagan vibes... So...
Sahmain/Halloween was traditionally the year's end, winter arriving and the thinning of the veil between the living world and the dead. Its the Stranger's time.
Midwinter is a time of bringing light into the short dark days, butchering those animals you couldn’t afford to feed, preserving food for the year ahead, repairing stuff, and a time to have fun. A festival of Light is common in a world where there is a dark winter. Eat, drink and be merry. It might not be pinned to any god or goddess, or it might be the Crone's time.
Spring (Imbolc) is the Maiden's time; things are being born, waking up, growing anew.
Ostara (Easter) is a time for renewal, the Mother Goddess is waking up.
Beltain (May day) is a time when things are getting together; animals are pairing off and 'a young man's fancy turns to thoughts of love'. Beltain is a fertility rite, and perhaps the Maiden and the Warrior might both be appropriate here.
Litha, midsummer, is a time of warmth, making, working. Maybe the Smith's time?
Lammas is the first harvests. Perhaps the Father is a good one here.
Mabon, the second harvests, things are fruiting, ripening in the hedges, a time of abundance. Perhaps the Mother again, or the Father and Mother.
Really, this is up to you. Writing in a fantasy world gives you leeway to decide for yourselves especially if it isn't already canon. Personally, I think you could add certain people into the festivals, like Baelor the Blessed, perhaps. The closest thing to a saint, maybe? So I rather think you can be creative as you like. Id like to know what anyone else thinks.
