What is Needfest? Well Greyhawk: The Adventure Begins says:
"This frigid seven-day period marks the transition from one calendar year to another, and is usually accounted as the start of the new year."
"All seven days of Needfest are public holidays in Greyhawk; this is a time to celebrate life when the weather is most miserable. Food stored away after harvest is brought out for grand dinners and all entertainment establishments are full to bursting. Each noble of Greyhawk invites his vassals and servants to dinner one night during this week, and charitable gifts of food and drink are common at such parties. The news and gossip of the year is reviewed and rehashed, and plans are laid for the coming year. Overindulgence and merrymaking are the rule. Some middle-class and noble citizens have a tradition of decorating their homes with yellow candles, boughs of evergreen branches (symbolizing the triumph of life through hard times), and even magical lights and ornaments. Daytime displays of harmless illusion/phantasm spells are held at the High and Low Markets in the Free City, drawing many spectators."
"Needfest is not a religious celebration per se, but local religions take note of it nonetheless. Special services are held at many churches and temples, complete with singing, music, sermons, and feasts to which the priests and their helpers invite all parishioners. The religions of Rao, Pelor, and St. Cuthbert banded together after the Greyhawk Wars to feed the refugees that flooded the city; this began a new practice that seems to have caught on."
"The evening of Needfest Godsday is also known as Midwinter Night. On this night, Needfest 4th, only aquamarine Celene is visible in the sky, as Luna is new. For this reason, Midwinter Night is also called Handmaiden's Glory or the Dim Night. This is a holy night for priests of Celestian, who conduct allnight services of moonwatching and stargazing on the grounds around the Grey College Observatory. Magical adjustments to the weather are sometimes made to ensure good viewing, though the temperature is always left cold enough to prevent widespread melting of snow and thus flooding. This tinkering with weather angers the few local druids, who consider this night holy and gather at the StoneRing outside the city's Druid's Gate for chanting, prayer, and other services."
Sorry about all the reading. I left out some of the boring parts in fact. As you can see it's a fairly close Christmas analogue and one that I run with each year. Expect more on Needfest from me in the next day or so.