Curiously, I'm not seeing the doouble FAQ.
It's also not just non-transfer days that need to be disabled. Currently it's offering Monday 12th to be selected. Yes, that is a transfer day, but it's too late to be selecting it now.
But with Easter coming up, giffgaff needs to urgently check that Good Friday and Easter Monday cannot be selected, as well as weekends -- and also needs to prevent selection of the Tuesday when the form is used after 3pm on Thursday.
A calendar widget probably isn't appropriate for this selection if it cannot be programmed to not show unavailable days.
I'm sure this used to be done with a dropdown picker, which is easier to control -- days that are not put in the dropdown cannot be selected.
But above all, the transfer form needs a very clear warning:
DO NOT START YOUR NUMBER TRANSFER UNTIL YOU HAVE FULLY TESTED YOUR SIM IN YOUR PHONE AND CONFIRMED IT IS ABLE TO MAKE AND RECEIVE CALLS
I'd suggest it goes immediately above Number you'd like to keep so that there's little chance of missing it.