Which day did Xuanzang choose as the auspicious day of the ecliptic to do the "land and water conference" for Emperor Taizong?

Xuanzang bowed to the imperial edict again, and then went to Huasheng Temple to gather many monks, build a monastery, arrange merits and arrange music. 1,200 monks, large and small, were selected and assigned to the upper, middle and lower classes. In front of the Buddhas, everything is orderly, and there are two times in the first place. Choose the third day of September this year, the ecliptic is so beautiful, and start the land and water conference on July 49 ~