"Onion snow" refers to the last snowfall of the winter season and is said to mark the beginning of spring. It received its name because onions are planted in early spring, and the later snow fall covers them, providing small portions of beneficial moisture.
The onion snow can be a light dusting or as much as 2 or 3 inches, but it usually melts by the end of the day. The phrase appears to have roots in rural Pennsylvania or the Pennsylvania Dutch culture. Most onions are planted within a few days of St. Patrick's Day, before the official start of spring on the vernal equinox. Other plants often planted around the same time include spinach and peas.