It doesn't matter when you learn it, so long as you learn it. A student’s grade should reflect her current understanding of the course, not last month’s, not her understanding when it was convenient for me to assess her. Keep a loose grip on your students' grades.
My assessment policy needs to direct my remediation of your skills. My comprehensive test on "Twelfth Night" won't do much for us two months down the road when you come in looking to patch yourself up. Assign separate scores to "Twelfth Night Themes," "Twelfth Night Vocabulary," and "Twelfth Night [whatever else it is you English teachers do]," scores which can be targeted and remediated individually.
My assessment policy needs to incentivize your own remediation. How many students will put in the effort to remediate their skills if the reward isn't tangible and immediate? Traditionally, what do you have? The promise that your studying here at lunch is really gonna pay off on the next test? Which is in three weeks. The student's like, awesome, glad I came in.