The term Onion Snow originated from the Pennsylvania Dutch. However, what it exactly refers to is up for debate. Some say the onion snow is the last snowfall of the season, and after it falls this is the best time to plant your onions. However, others say it is a snow that falls on recently planted onions.
An Onion Snow is usually known to be a lighter snowfall compared to a big winter storm. On average, we usually see our last snowfall of the season in the month of April, but it can be as late as May.
The best time to plant onions in Pennsylvania is in late March or through Mid-April.
In any case, the onion planting season is very close to when we typically see our last snowfall of the year, which is probably how this term came to be.