(And it never has been. 'Oh')
In 1644 Christmas day fell on a legally mandated day of fasting. Not dependent on an exclusively ‘mince pie’ diet, the population of England at the time simply weren’t allowed to eat anything. You see, when they were allowed to eat again, they could eat – well – anything really, and yes of course, mince pies. It’s just that they would have to wait until the Christmases of 1645 or 1646 to enjoy one on Christmas day, although things became a little tricky in 1647!
The Long Parliament (it went on for about 20 years - AKA the Long Parliament Of the Interregnum because it covered the period after the death of Charles I until his son Charles II was restored) abolished not just Christmas, but all festivals that year in the imaginatively named “An Ordinance For Abolishing Of Festivals 1647”. But if I decorated your entire house with a solitary mince pie whilst you might ‘think’ of Christmas it hardly ‘is’ Christmas. Once again the humble pie itself escaped targeted legislation.
It got a bit trickier 9 years later in 1656 when further legislation was approved to curtail illicit Christmas celebrations. In fact, you could argue that during that intervening nine year period it was a little tricky and perhaps frowned upon, if that nibble might be the beginning of your journey to an illegal festival, but again, it was never illegal on its own.
It’s probably briefly worth considering that until the mid-Victorian period if you enjoyed a mince pie you would have found yourself with a mouthful of a good deal of actual minced beef. But by 1861, even Mrs Beeton only featured beef in one of many variations of the reciepe. The long and short of which is that if there was a law - which there wasn’t - you’d hardly be breaking it by eating something different. (Unless it was the Christmas of 1644 when you couldn’t eat anything of course.)
Luckily there can be little doubt now. For when the monarchy was restored, the 1647 Act itself never was, as due to a headless king and it not having received Royal Assent, it was never re-enacted. Hurrah!
So good news, you can feel safe that you can not only celebrate Christmas, but enjoy a suitable volume of mince pies, whether with the addition of beef or not!
Just be a little careful in the manner in which you receive your helping from the platter of smoked salmon, unless you want to be convicted of ‘handling salmon in suspicious circumstances’.