If there's any rhyme or reason to their method, I sure can't discern it! Sometimes a cancellation lottery will be held for a launch date less than a week out (example: there was a lottery on 3/5/2015 for a 3/9/2015 launch!), sometimes it goes to a call in around two weeks before launch, and sometimes it doesn't get filled.
I agree with Buckmanriver that 2016 cancellation offerings seem particularly sparse so far. I thought for sure we'd have a lottery last week. Didn't happen. Thought we'd have one this week. Didn't happen. Good thing I don't have any money riding on this!
And when cancellation dates have come up in the past two lotteries (held on 6/23/2015 and 10/15/2015) there was not a single Feb date available and only a couple of March dates. Are cancellation rates going down all of a sudden or is the next lottery going to have a lot of dates? Seems kinda weird, but there's nothing to do but wait.