When the Jews came up to build the Bais Hamikdash certain families brought wood for the bais hamikdash. Those families got this mitzvah forever.

The time for bringing wood was considered a festival. It was done 9 times in the year:

1. The first day of Nissan—ben Arach ben Yehuda
2. The 20^th of Tamuz—ben David ben Yehuda
3. The 5th day of Av—bnei Parush ben Yehuda
4. The 7^th of Av—bnei Yonadav ben Reichav
5. The 10^th day of Av—bnei Siah ben Binyamin
6. The 15^th day of Av—bnei Zeitu ben Yehuda

Those who did not know which tribe they belonged to and cohanim and leviim brought now. Also the families of pestle smugglers & fig cutters. [In times when they did not let them bring Bikurim these families tricked the sentries.]

7. The 20^th day of Av—bnei Pachos Moav ben Yehuda
8. The 20^th day of Elul—bnei Adin ben Yehuda
9. The first day of Teves—bnei Parush family brought again. This day there was no maamad since there was Hallel, Mussaf and a korban of wood.

Taanis 4:6

Five things happened on 17^th day of Tamuz:

1. The tablets were broken
2. The korban tamid stopped
3. The wall of Yerushalayim was broken
4. Afpasmos burned the Torah that was found in the Azara
5. Manasheh put an idol in the kodesh

Five things happened on the 9^th day of Av:

1. It was decreed that Bnai Yisrael of age would not go into Eretz Yisrael
2. The first Bais Hamikdash was destroyed
3. The second Bais Hamikdash was destroyed
4. The city Beitar was destroyed
5. The city Yerushalayim was plowed under

When Av starts we lessen happiness.