*Chuckles* It doesn't have to be 104th day. And there can be more reason to it than "merely an excuse for a party".
It could be that it was not only the 104th day of the year, but also the first day that a sick child of the former Digni leader was healthy and not in danger of dying. Or it could mark the day a siege was broken and a particularly bad conflict ended.
There could also be 171st day, 225th day and 309th day.
As for the three sisters, they are:
Teacher - a winged Cougar, the cheery one. She has a brightly colored golden wisp with red feathered wings
Healer - a blue Koyoti (blue fur with white underside, thin black stripes, long white hair with black tips, blue feathered wings with black tips, green eyes). She has a blue and white wisp. (hooded like a cobra with black stripes and blue feathered wings with black tips)
Warrior - a crimson Ram with long golden horns (despite being female) (red with a white muzzle and underside, thin black stripes, red leathery wings, green eyes and golden hooves). She has a red wisp of the variety that have bat like wings.
The decorations for each holiday would likely be similar to each sister's colors, and the holiday would likely be a reflection of how each one would likely enjoy their down time. They would also likely be times to celebrate careers that exemplify each of the Sister's philosophies.
Tiffany recommended that Zyra's Day be a holiday that centers itself around drinking, eating lots of meat and partying all day and night. Which both I and Renessa (And I think Shippou) were all for. Guards, Soldiers and Military would be the celebrated careers. Red and gold decorations would likely be the theme.
Zylac's Day would likely center around just having fun and being with friends. The preferred gifts would likely be books or trips to somewhere the receiver wouldn't normally go, to expand their knowledge. Tans and earth tones would likely permeate her holiday. Teachers and wise elders would be the celebrated people/careers.
Zyphir's Day would probably be placed opposite Resolution and be dedicated to celebrating the healers, medics, etc. It's possible that it would be dedicated to forgiveness and healing old wounds of the figurative kind, but that might be redundant given what Resolution Day is.