Mars Day is the name of a Colonial weekday. It is named after the Colonial God of War (Occupation).
Note
The use of the name "Mars" is paradoxical with Colonial religious history in the Re-imagined Series, as Mars is the Roman name of the Greek god Ares after the Roman assimilation of the Greek pantheon and the change of names to these gods (Zeus to Jupiter, Hera to Juno, Athena to Minerva, Ares to Mars, et al.) This is the second instance where Roman pantheon names have been used in the series: Pegasus is a Mercury class battlestar.
In french, "Martis dies" (literally, "Mars' Day") is Tuesday. The english name derives from Tyr, the Norse equivalent of Mars.