In most cases the sitting is decided by the timetable. The course runs first semester, so the exam is written in January. Nobody treats it as a choice.
It is worth treating as one, because the sitting changes two things that matter: how much runway the student gets before the exam, and how much room is left to do anything about the mark afterwards.
Alberta offers diploma exams in more than one sitting a year, and the exact dates and the rules for registering, rewriting and getting a mark reconsidered move. Check the current schedule and the current rules on the Alberta Education site or with the school before planning around any of it — the shape below holds, the dates do not.
Runway. A first-semester course ends in January, which means the exam lands immediately after the content finishes and in the same stretch as every other first-semester final. A second-semester course ends in June, with the same compression at the other end of the year.
Room to recover. This is the one families underestimate. A mark that comes back in February leaves the rest of the year available. A mark that comes back in July does not, and anything that follows it happens over a summer, with deadlines that belong to somebody else.
What else is happening that month. A student with three diploma courses in one semester writes three diploma exams inside a couple of weeks. The same three spread across two semesters is a different year.
Admission offers for the following September are frequently conditional, and conditional on final marks that arrive after the June sitting. A January mark is a known quantity while applications are still being put together. A June mark arrives after most of the decisions have been made and has to be fitted in behind them.
That cuts both ways. A strong January mark is evidence in hand. A weak one is a problem you have time to address, and that is the more common and more useful case.
Scholarship deadlines run on their own calendar and often earlier than the admission ones. If a particular award matters, find its deadline before deciding anything, not after.
Put the strongest course in the sitting that comes first. It needs the least preparation time and it gets a confirmed mark on the record early.
Give the shakiest course the longer runway, and use the runway. Choosing the later sitting and studying in the last three weeks of it converts the only advantage into nothing.
Do not stack three diploma exams into one fortnight if the timetable can be built any other way. Ask the counsellor early — this is a scheduling conversation that happens in spring for the following year, and it is largely over once the timetable is built.
If a rewrite is a realistic possibility, work backwards from when the second sitting is and when the mark from it would land. A rewrite that arrives after the deadline it was meant to fix is a lot of work for nothing.
The sitting is worth a few marks. What the student does in the weeks before it is worth considerably more, and the biggest difference between the two sittings is usually not the date — it is that a January exam quietly removes the Christmas break as study time, and a June exam has to compete with every other thing happening in June.
Whichever way it falls, the exam is a known date months out. The students who do well on it are the ones who started from that date and worked backwards, rather than noticing it three weeks out.
Give the app the exam date and it builds the plan backwards from there, weighted toward the topics that have been costing marks.