My first semester teaching I had a few students who "could not make" an exam". The next semester, and subsequent ones, I informed students that they could miss an (1) exam if they wished. However, I informed them that the make up exam would be more detailed and much more rigorous. Subsequently, in the 3 semesters that followed, 0 students missed exams.
That may not always be the case for you, but I suspect that if you provide a clear and detailed rational for students as to why they should not miss an exam, most will figure out a way to attend. Just my experience, which could certainly differ for you!