Under NEC3 ECC Clause 51.2, each certified payment is made within three weeks of the assessment date. So, if the assessment date is 1 May 2019, what is the payment due date ? (1) 21 May 2019, or (2) 22 May 2019. (Calculation of payment due date, whether the assessment date is day 0 or 1)
Using your example and the NEC timescales, if the payment assessment date is 01 May 2019, the payment due date would be 08 May 2019 and the final date for payment is 22 May 2019.
Interestingly, ‘May Day’ is a public holiday and NOT a bank Holiday, otherwise the above timescales would not include this day in the calculation.