You can't be serious, it's a silly idea from just about any POV. No government in their right minds would introduce such a thing, it would make a rod for their own backs (people with long memories might remember the short-lived Tax and Prices Index which was supposed to replace the RPI to measure the cost of living including taxes, but which was quietly forgotten once they realised they couldn't actually cut taxes).
Also as a practical matter it would be pretty bad to have a holiday which moved around every year and which potentially wouldn't be decided until just before it happened (since the budget is in March). And what if it falls at a weekend? And intrinsically it makes no sense, tax rates are wildly different for different people, and there is no clear definition of how you decide what counts as tax, e.g. do tax credits and allowances get treated differently from benefits? Do corporate taxes count, even though they may be levied on overseas shareholders?
Also of course there's the slight inconsistency between being in favour of lower taxes, and opposing university fees. As I say I think MH is just being opportunist, he doesn't expect to ever be PM so he can promise what he likes. Personally I think it would at least present a real choice if we had a party which was genuinely committed to significant tax cuts, but since in practice that would mean significant privatisation of health and education I doubt anyone will actually propose it. Are the Tories even going to propose rolling back the Labour spending increases they appear to be complaining about? Somehow I doubt it ...