When does the three weeks start with regard to music and shaving, on Monday night or on Tuesday morning?
The Halachos of the three weeks start from the night of the 17th of Tammuz. (The fast should technically also start from then, but chazal weren’t gozer on it because it is too hard.)
Shar Hatzion 565-8, Chida in Chaim Shoal 1-24, Ze Hashulchan (R’ S. Doblivsky shlit”a) 2 Siman 551-1, Orchos Rabeinu 2 pg. 159.