The ninth month of the Islamic calendar "year". It is a holy month and, during it, Muslims are forbidden from eating, drinking, smoking, and sex during the daytime.
This month can come at any time of the Gregorian calendar year. In Gregorian year 2005, Ramadan begins in early October. In AD 2011, Ramadan will begin about when August begins.
Jamal isn't going to eat lunch with us today because it's Ramadan.