SB 8.16.48
pratipad-dinam arabhya
yavac chukla-trayodasim
brahmacaryam adhah-svapnam
snanam tri-savanam caret
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
From pratipat until the thirteenth day of the next bright moon [sukla-trayodasi], one should observe complete celibacy, sleep on the floor, bathe three times a day and thus execute the vow.