If you signed up two weeks before the end of the current month, your Diesel card will typically arrive near the month's end, ready for use from the first working day of the following month. 

If you signed up within two weeks of the month's end, your subscription will commence the following month, meaning you'll receive your Diesel card near the end of that subsequent month.
This is due to the time required for card creation, personalisation, as well as the processing, postage, and packaging procedures.

For example, if you signed up between March 1st and March 17th, your Diesel card will arrive near the end of March, ready for use from the first working day of April.