According to legend, if the weather is overcast when the groundhog emerges from his burrow on 2 February, spring will come early. If it is sunny, the groundhog will see his shadow and winter will last for 6 more weeks.
Memorial Day and Labor Day are sort of the bookends of summer. The former marks the beginning of summer; the latter, summer's end. But why don't the Americans celebrate Labor Day on May First?