The 3 week appointment time shift into the future is regularly mentioned on here when an installation is delayed.
I doubt VM's order management system is that sophisticated to factor in bank holidays and the date has just been bumped on 3 weeks by an automated process.
Your story has all the tell-tale signs of a lengthy installation delay in the offing. Read through this forum and you will find many similar stories.
Are you waiting for a new cable to be installed to your property?