The Winter Solstice is Monday, December 21 at 12:47 p.m. EST. This date is regarded as the "start" of Winter for the Northern Hemisphere, and is also the shortest day of the year (i.e., with the fewest hours of sunlight). The Earth's axis is always tilted at 23.44 degrees with respect to the orbital plane, which means that different latitudes will most-directly receive the Sun's incident rays at different times of the year on the Earth's orbit. 
From an observer's perspective on Earth, it appears that we seem to "shift" either toward or away from the most direct rays during the year. In actuality, we aren't tilting back and forth from the Sun's perspective, but from ours on Earth, that's what it looks like. 
By the Winter Solstice, the Sun's incident rays are so low on our southern horizon that much of their intensity is diffused as it travels through a longer layer of atmosphere to reach the ground in the Northern Hemisphere. The good news: our days will begin lengthening again. The "bad" news: the temperature lag on Earth means that we are just entering our climatologically coldest months. Hence, why we call this the "start" of Winter!
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