First Direct Debit Payment

Hi Guys,
I’m due to be transferred over to Bulb around the 26th of this month from my current supplier, and I would like to know when would the first DD go out of my bank? Is it the case that when I send them my reading they take or does it work one month in arrears?

The first DD will go out on switch day, and then the same day of the month thereafter.
The initial reading(s) (plus or minus 5 days from switch date) will also be sent by Bulb to your old supplier as final readings.
It’s one month in advance.