Shabbos Nachamu
Sefer Minhag Tov (first printed in 1929/ט"תרפ from an old manuscript by an Italian gadol) writes, “One shouldn’t take a haircut before erev Shabbos Nachamu, and when erev Shabbos Nachamu arrives, one must take a haircut, and one must be happy, and to welcome Shabbos with joy and a good heart, and he will be consoled with many consolations, and Hashem will console us with the redemption of Yerushalayim.”
Similarly, the Mahari'l states that on Shabbos Nachamu בנחמת ויבטחו העם כל ישמחו הגואל, "The entire nation shall be happy, and trust in the solaces of the redemption."
On the Mishnah (Taanis 26:) that states, היו לא באב עשר כחמשה לישראל טובים ימים, "There weren't yomim tovim for Yisrael like the 15th of Av", the Ritvah writes, בשבת סעודה עשות נהגו זה ומפני באב תשעה שאחר, "Therefore, there is a minhag to have a special meal on Shabbos after Tisha b'Av."
שועיב בן, student of the Rashba, writes, טוב כיום לעשותו מצוה, it's a mitzvah to celebrate it like a yom tov.
