There are other dates, as well: the nomination deadline date and dates that have to be complied with. If you're doing it over the Christmas season, there are going to be days that are going to be problematic for Elections Canada to find staff and to have offices open.
I recall that in 2005-2006, Elections Canada's offices were open on Christmas Day. It's problematic when you have to find people to staff those offices at that point in time. It's a range, an extra seven days, and I think it provides some flexibility when those situations happen in minority parliaments, and there could well be minority parliaments in the future.