Today, Friday and Saturday are the three ember days of Lent this year: the spring ember days always fall the week following Ash Wednesday. These days of voluntary fasting and abstinence are set aside by the Church to focus on God as revealed through Creation.
The observance of ember days, four sets of three days throughout the year set aside for fasting and reflection on Creation, comes from ancient Israelite practice:
Thus saith the Lord of hosts: The fast of the fourth month, and the fast of the fifth, and the fast of the seventh, and the fast of the tenth shall be to the house of Judah joy, and gladness, and great solemnities: only love ye truth and peace.
In the mid-1200s, Bl Jacopo de Voragine set forth the various reasons he was acquainted with for the ember day fasts. He said that the Lenten ember day fast should be for the intentions of tempering our inclinations to luxury, of casting away vice and letting "boughs and herbs of virtues" grow in us, of suppressing disordinate concupiscence, of rooting out pride, and of growing in innocence.
Medieval folklore said that the weather of the ember days predicted the weather of the coming months, thus today's weather would foretell March, Friday April, and Saturday May. We're going to hope that this doesn't pan out, because we still have several feet of snow and are well below 0!