Dr. Alan Kurschner explained that the expressions the “day of the Lord” and the “second coming” (Parousia) refer to the same event, the return of Christ. They are co-referential and thus are used interchangeably.
Dr. Kurschner cautioned against committing the word concept fallacy, where one would require that a particular term must be present in a text for a concept to be described there. The biblical writers had the freedom to use many different terms to describe a single event, such as the return of Christ.
He gave several specific biblical examples that demonstrate this. For example, Peter and Paul use the expressions “day of the Lord” and the “coming” (parousia) interchangeably in various contexts, such as events leading up to the second coming or our spiritual posture leading up to the second coming.
See Dr. Kurschner’s most recent book, where he discusses this issue: