There are principles behind this:
- The abbreviation starts with the first letter of the English or French name. (By chance, English and French always agree on the first letter. Not true with, for example, English and Spanish: January/enero.)
- The second letter has to be in both the English name and the French name. Consider the month April/avril. It can't abbreviate as "AP", because there's no "P" in "avril", and it can't abbreviate as "AV" because there's no "V" in "April".
But both "April" and "avril" have "L" in common, so they can (and do) abbreviate as "AL".
(The other possible solutions: "AI" and "AR".)
Incidentally, from these, you get that the
only possible abbreviation for May/mai is "MA", because the "m" and the "a" are the only letters they have in common. So, May/mai has to be "MA". And because "MA" is taken by May/mai, then March/mars can't use it, so, March/mars must be something else. The only possible abbreviation, once "MA" is already taken, is "MR".