To repay her kindness and to benefit the gods, Buddha spent three months teaching in the Heaven of 33. This is not as often stated, in Tushita heaven; Tushita is the realm where the Buddha lived before he descended to Earth. Possibly somewhat confusing can be that one week before, on the 15th of the 9th month, it is celebrated that he accepted to return from the Heaven of 33.
Buddha Shakyamuni's Descent from the Heaven of 33 - Lha Bab Düchen
The 22nd of the 9th month, 'Buddha Shakyamuni's Descent from the Heaven of 33' is celebrated. Buddha's mother had been reborn in Indra's heaven.