The days between the 17th of Tammuz and the 9th of Av are days of mourning, on account of the collapse of Jerusalem during the Roman occupation which occurred during this time period. Weddings and other joyful occasions are traditionally not held during this period. A further element is added within the three weeks, during the nine days between the 1st and 9th day of Av - the pious refrain from eating meat and drinking wine, except on Shabbat or at a Seudat Mitzvah (a Mitzvah meal, such as a Pidyon Haben - the recognition of a firstborn male child - or the study completion of a religious text.) In addition, one's hair is not cut during this period.
(Adapted from Wikipedia)