The fast of Shiva Asar B'Tamuz is on Tuesday, July 23rd, and marks the beginning of the Y'mei Bein Hametzarim, the Three Weeks.
The Y'mei Bein Hametzarim, between the 17th of Tamuz and the 9th of Av, is a national mourning period over the destruction of both Batei Mikdash.
One should focus on the churban and exile during this period. Activities restricted during this period include:
Music and Dancing: Children of chinuch age are included. Many poskim are lenient when the music is secondary (e.g., background music on a story CD) or when the listening is not for enjoyment (e.g., to help one stay awake while driving). Multiple poskim are stringent regarding acappella, "sefirah music." Playing and practicing music are permitted to earn a livelihood (e.g., professional musician). Playing music to develop one's skill is a matter of dispute amongst the poskim.
Haircuts and Shaving: This prohibition applies to men, women, and children. However, in cases of discomfort, many permit women to tweeze and remove hair from areas other than the head.
Weddings: Weddings are not held during this period. An engagement may be celebrated, although without dancing or music. A Sheva Berachos may be held without music, although dancing (and singing) is permitted.
Shehecheyanu: We avoid situations that would necessitate reciting the berachah of shehecheyanu (e.g., eating new fruits, etc.). Miscellaneous: One should consult a Rav regarding signing a contract for a new home, moving into a new home, decorating a house, and elective surgery.