When to submit regular meter readings

I wondered which day of the month should meter readings be submitted? Is it a fixed date (eg. Last day of month) or a certain number of days before the direct debit is taken out? Thank you.

Your first statement will be issued a calendar month after your switch.
You should get a reminder email 3 days before the billing date.
Submit the readings on that day, or the day after, and your statement will then be based on readings rather than estimated.