Citing from here
There is a famous Mishnah that is cited towards the end of Masechet Ta’anit (4:6) that states:
“Mishenichnas Av, Mema’atin Besimcha”
“When the month of Av arrives, we decrease our happiness”.
It seems that once Tish'a B'Av is over (well, in a sense, the afternoon of the 10th) joy increases significantly. People resume eating meat and drinking wine; people bathe / swim; people listen to music. Essentially, it seems that people resume all activities that they did prior to the start of the Three weeks.
Is there anything that says or implies that the statement from the Mishnah doesn't apply to the entire month? It does explicitly state when to start, namely the beginning of Av. But, there is nothing that states when the lessening of joy should end.