You are correct in this way: in the use of 'pesach'/passover in the Pentateuch, it refers not to a day, but a sacrifice sacrificed in the afternoon of the 14th daytime.
In later writings, such as the New Testament, 'Pesach'/'Pascha'/'Passover' is also used as a popular title for 'The Festival of Unleaveneds', which is not merely one day, but a whole week long.
Christian groups, such as SDA and WCG derivatives have thus wrongly asserted that the 14th day is called 'Passover', and from this error have come to a wild conclusion that it might occur in the night before this day. They are in error, but think that Yeshua's Passover meal the night before the Jews' proves them correct. That is false. Both the Jews and Yeshua would have known that Passover was eaten in the night after the 14th daytime. That in the year and month of his crucifixion they differed in their determination of which day was the 14th, is not difficult to conceive, and provides a better explanation of the apparent discrepancies in the Gospels, than the Christian assumption above, and the assumption that the Gospels are incorrect.