I believe this product is UK Based, where the first day of the week is MONDAY. I would like to think I'm not the only potential customer in the US of A. Here, the first day of the week is SUNDAY on calendars.
Please offer the ability to pick the first day of the week. Perhaps the users' country (which you should have in their payment info) can help inform the default option.
Here is an example calendar which i'll say is wrong but someone in another country would say is 'just fine'.

-Rob