First of all, the time: WB has cocked up here. Time wasn't standardised across the country until the coming of the railways, and it certainly wasn't so precisely measured 800 years ago that we can say something had happened at exactly 12.15pm. The 12.15pm time is obviously to reflect the year the Magna Carta was signed: 1215 AD, which is a bit embarrassing for our leading real ale newspaper.
Secondly, the relevance of the Magna Carta is that article 35 states:
Let there be throughout our kingdom a single measure for wine and a single measure for ale and a single measure for corn, namely the London quarter and one width of cloth, whether dyed, russet or halberjet, namely two ells within the selvedges. Let it be the same as with weights as with measures.
Yes, it mentions beer, but it's not really about beer; it's about weights and measures, but that's okay too because it shows that CAMRA magazines printing the contact details of local trading standards have an 800 year old precedent.
I don't mind campaigns like this and "Let There Be Beer", even if I do think the latter is a bit naff, because they depict beer as something normal and not a scourge on society causing uncontrolled mayhem and disorder. I just wonder how far the general public were aware of National Beer Day when someone like me who is interested in real ale completely missed it?