Shabbos Graphic #7

(b) The pattern of letters, which stand for Shabtai (Saturn), Tzedek (Jupiter), Ma'adim (Mars), Chamah (the sun), Nogah (Venus), Kochav Chamah (Mercury) and Levanah (the moon), starts at the first hour of Wednesday evening (i.e. the hour in which the heavenly hosts were put into place). They are in descending order, from the farthest body from the geocentric earth to the nearest (Saturn is the farthest and the moon is the nearest. In a geocentric orbit, the sun takes the place of the earth.)

(c) As the Gemara explains, Ma'adim "rules" by day during the following even hours:

1) 14 (2nd hour of the day) - on Thursday. It is not advisable to let blood that day, since the red planet, Ma'adim, is the Mazal of evil and spilled blood.
2) 16 (4th hour of the day) - on Monday. It is not advisable to let blood that day.
3) 18 (6th hour of the day) - on Friday. The Gemara advises (Avodah Zarah 29a) that one should eat fish after letting blood. The Gemara states (Shabbos 118b) that one should eat fish on Shabbos. Many people therefore made a habit of letting blood on Friday. Once an action becomes commonplace, it is no longer affected by Mazikim.
4) 20 (8th hour of the day) - on Tuesday. It is not advisable to let blood that day.
5) 22 (10th hour of the day) - on Shabbos. Blood letting is of course prohibited on Shabbos.
6) 24 (12th hour of the day) - on Wednesday. It is not common for people to let blood so close to evening, and therefore the Chachamim did not advise against blood letting on Wednesday (TOSFOS DH d'Kai).