En España usamos LMXJVSD, con una X para el miércoles y asi no se confunda con la M de martes.

Im from Mex, and I have never seen that x in the calendars. And Monday is the most of the time the first one Usually would be like that D L M M J V S Regards!

There are two types of abbreviations. Depending on the country you could use one or the other. Case1: L K M J V S D Case2: L M X J V S D In Costa Rica we use the letter K to indicate Tuesday

I often see: L, M, M, J, V, S, D in Spain calendars. Windows and many popular calendars use: lu, ma, mi, ju, vi, sá, do, respectively. I've never seen the usage of X to short wednesday in the normal calendars, but it is needed when you don't want any kind of uncertainty about if M stands for Martes or stands for Miércoles, so the X is used for Miércoles (when one character is used). There are quite a few theories about why the X was used for Miércoles, but that's a story for another day. I hope this helps you out. Take care!