Today is Nisan 13th 5771. At sundown this evening the date will change to the 14th of Nisan. Jehovah's Witnesses will celebrate the Memorial this evening.
Tomorrow, it will still be the 14th. Nisan 14th will extend throughout the entire daylight period on Monday clear to sundown. The Torah is emphatic that the next day after Passover is the 15th, not the 14th.
"And they proceeded to pull away from Rameses in the first month on the fifteenth day of the first month. Directly the day after the passover the sons of Israel went out with uplifted hand before the eyes of all the Egyptians." (Numbers 33:3 NWT)
"They set out from Rameses in the first month, on the fifteenth day of the first month. It was on the morrow of the passover offering that the Israelites started out defiantly, in plain view of all the Egyptians. (Numbers 33:3 Tanakh)