"Av was once a joyous month." (source)

"When Av comes round, one cuts down on merriment." (Glinert)

"As the Hebrew calendar flips from the month of Tammuz to Av, it is a good time to begin to prepare [for the high holidays]." (source)

"A large portion of Av is spent lamenting for the sins of the past and petitioning God to have mercy in the future." (source)

"As you can see, the month of Av ebbs and flows with the themes of destruction (judgment) and renewal." (source)