Midwinter Festival is similar to christmas, but has more to do with natural, rather than divine occurances.
It occurs on the shortest day of the year, during winter. It is the strongest period for the gods who have darkness and evil type domains, but is a time of rejoicing for other worshipers, as the sun (and with it associated domains and powers) would be growing stronger throughout the year.
Druids and their ilk often perform ancient ritual at this time of year to ‘bring back the sun’. They know it isn’t technically necessary, but perform this ritual (especially in Sharn) as a demonstration of the religion of their people.