Has any one any idear how often Machinemart has there VAT free days

I get a thing in the post about 2 or 3 time a year

but they actually do it more often than that as they send them out to 3 or 4 groups of people with different dates (me, my dad and one of my mates all get them, but they are never the same days)

I purchase from them for myself, and for work. I get the Vat free offers at different times to my different accounts. Dont know if it depends on when/if you have spent money with them recently?