<!-Quote-!>Quote:Winter solstice- when the night and day are the same length.<!-/Quote-!>
That's the definition of an equinox. The Winter Solstice is the day on which we have the least hours of daylight, also marking the beginning of winter.
On the current Gregorian Calendar system, the winter solstice tends to occur either on the 21st or 22nd of December. However, on the old Julian Calendar, the one which was around when Christmas was invented by the Romans (adapting events such as Saturnalia, and prior pagan festivals), the solstice did indeed fall on the 25th December.