The meaning of words can change over a period of time especially over hundreds of years. Passover may have been first used for one day, but the term later was used for that one day and the seven days of the feast of unleavened bread which followed that one day. Each of the two verses need to be interpreted in their own context to understand the meaning of its words when written.
Comparing Scripture with Scripture, Luke, who was also the human writer of the book of Acts, clearly used the Greek word pascha to refer to either the entire period--the one day of Passover and the Feast of Unleavened Bread or as an acceptable name for the Feast of Unleavened Bread. Luke wrote: “Now the feast of unleavened bread drew nigh, which is called the Passover” (Luke 22:1). At Ezekiel 45:21, the Passover is referred to as “a feast of seven days.” In Ezekiel 45:21, the name “Passover” was clearly used for or used to include the feast of Unleavened Bread, which is a feast of seven days. In Matthew 26:17, the name “Passover” was used for a time described as “the first day of the feast of unleavened bread.” After Jesus and his disciples had already observed the feast of Passover (Luke 22:14-15; John 13:1), the same Greek word was still used for a time when the feast of unleavened bread was in progress (John 18:28).