Saturdays, Sundays, Christmas Eve, Christmas Day, Maundy Thursday, Good Friday, bank holidays and any day appointed for public thanksgiving or mourning are disregarded in the timetable. A bank holiday for a by-election is only disregarded if it is a bank holiday in that part of the United Kingdom in which the constituency is situated.
The 15th day after the writ was issued would have been the 11th or 12th of August and the 19th day would have been the 15th or 16th of August. Checking these things isn't hard, but Labour would appear to prefer just making it up as they go along.
For the sake of completeness of information, there is no rule that enforces the holding of a by-election during a Parliament:
There is a convention that the writ should be moved within about three months of the seat becoming vacant, but this is not a statutory or parliamentary requirement.
Who'd bet against Labour just holding out until the General Election next year?
Mind how you go!