With weekday(date, 2):

Monday = 1

Tuesday = 2

Wednesday = 3

Thursday = 4

Friday = 5

Saturday = 6

Sunday = 7

We want to calculate how many days to add to the 1st of the month to reach Monday.

mod(8 - weekday(local!firstDay, 2), 7)

For example:

If the 1st is Monday:

mod(8 - 1, 7)

= mod(7, 7)

= 0

So Monday + 0 = Monday.

If the 1st is Tuesday:

mod(8 - 2, 7)

= mod(6, 7)

= 6

Tuesday + 6 = Monday.

If the 1st is Sunday:

mod(8 - 7, 7)

= mod(1, 7)

= 1

Sunday + 1 = Monday.

The result is:

{

01/05/2026,

02/02/2026,

03/02/2026,

04/06/2026,

05/04/2026,

06/01/2026,

07/06/2026,

08/03/2026,

09/07/2026,

10/05/2026,

11/02/2026,

12/07/2026

}

Key formula to remember:

mod(8 - weekday(date, 2), 7)

This gives the number of days needed to move from the 1st of the month to the first Monday.