They also didnt want a holiday town full of people from 2 towns that were close. Can you imagine a drunken fight between Accrington and Blackburn mill workers?There was. Different towns would have different weeks.
There was a further benefit to this. The mills would use the shutdown for heavy maintenance, often using specialist contractors. Clearly these contractors couldn't be everywhere at the same time so the wakes weeks would be spread across July and August. It also meant that school holidays varied between towns. When I first moved to Crewe I discovered that our summer holiday was two weeks at the beginning of July, back to school for a fortnight then the whole of August off!