So for weekly payrolls you can have 53 weeks, fortnightly 54 weeks and 4 weekly/lunar payrolls can have 56 weeks rather than the normal 52.
It isn't very common either and in the case of 4 weekly payrolls it only happens once every 22 years.
One of the other peculiar features is that the tax code is allocated cumulatively over your normally year so by the time you hit week 52 it is fully used. If you have any extra weeks beyond week 52 you will be taxed using an extra allocation of free pay.
So a 4 weekly payroll will receive 14 / 13ths of your tax code. To a higher rate tax payer that could be worth £230 of extra pay above what you would normally receive in a tax year. So in effect you get more personal allowances than you are allocated in a year.
Unfortunately, if you need to complete a tax return or HMRC work out your tax another way this will be clawed back in the calculations. You are obviously only entitled to your personal allowance each year but it seems that if you don't have to produce a tax return HMRC will never pick up it up!