It goes without saying that exams take place after, rather than before, relevant material has been covered in a course. To be sure, many of us use pre-tests or other diagnostics to find out where our students are at before we embark on a new concept, but the "tests" we use to measure student learning don't make sense unless they come after the learning has taken place. In that respect, the timing of exams has more to do with finding natural stopping points for learning (often driven by the semester calendar) than anything as radical as rethinking the temporal relationship of first learning material and then being tested on it.
On a deeper level, however, timing is a crucial consideration when designing the global and more local aspects of a course, because learning is a process for which order matters. Not only is that true at the conceptual or thematic level, as we decide to place one lecture or reading or problem set before or after another, but it's also true at the level of smaller arcs within a course, as we figure out the best way to sequence lower-stakes assessments to help students track their progress leading up to higher-stakes assessments, namely, the mid-term or paper.
When we think about course design in terms of timing and sequencing, we often refer to the concept of scaffolding, a metaphor that tries to capture the importance of providing students with low-stakes opportunities to practice—and fail at—using individual new skills alongside the skills they've already learned. Of course, the already learned skills were themselves practiced and honed in previous settings, and the new skills are building toward other, even more complex skills—all in the service of allowing the exam or term paper to play its role as a measure of the learning objectives students have been working toward, step by step by step. In this way scaffolding leads us to think not only about the skills or content students need to know for the test or the paper—but about how the order in which they experience the acquisition of those skills and content has a defining effect on what those skills and content, in fact, are.