To give a bit of life to George's answer, I'll tell you about my BMQ/SQ/trades course.
I did weekend BMQ and SQ. About a third of the way through my BMQ I asked if there was going to be a weekend SQ after it. I was told by the course officer that no, it didn't look like it, that there were certainly no plans and I'd probably end up doing it in the summer. (This would make my training take two years, not one).
The second to last weekend, they asked for names for the weekend SQ course that was starting the weekend following our grad parade. Of course I put my hand up. It still didn't mean I was on the course. The next parade night they told me I was on the course. The parade night before the SQ was to start, they told us the start date had actually been pushed back to the following weekend, but we still needed to go in for a half day the coming Saturday to do some start-of-course paperwork. We showed up, and it turns out we had been misinformed - we didn't need to be there, and weren't expected until the following weekend.
For my trades course, I was given dates at the end of July. I booked the dates off work. Then I was told that I wasn't actually on the course, but was either #1 or #2 on the "wait list" depending on how many spots were on the course. I found out two weeks later the entire course was cancelled, and a new course, starting a week later was created. This created problems for a few people, and so I was loaded on to the new course. At the same time, I knew people who were supposed to be going to Shilo at the start of July. Their course was also cancelled and they ended up being put onto the course at the end of July that was cancelled, but now was not.
So - in short- it all depends. Staffing is a big issue, especially with combat arms trades. The higher up the ladder you go, the fewer people you have able to teach, especially with the operational tempo the way it is. My unit ran a BMQ in October in 2005 and 2006. Not last year, and maybe not this year. In fact, this training year (07-08) is the first in a few years where no unit in the lower mainland has run either a BMQ or SQ on weekends.