Most people associate St. Patrick's Day with green beer.
Gardeners associate it with green peas.
March 17 is the traditional day for planting sweet peas -- the vegetables, not the flowers. Peas can be planted as long as the soil is workable and not too wet. They'll sprout in temperatures as cool as 40 degrees.
Check out this video from Fine Gardening's Vegetable Gardener e-newsletter for pea-planting how-tos.