The (American) holidays marked on the calendars are:

-Flag Day
-Columbus Day
-Christmas
-Thanksgiving
-April Fool's Day
-New Year's

puzzle asks you to put everything in chronological order. However, you might notice that they already are in chronological order by month. So what could this mean? Careful inspection should reveal that the calendars are from *DIFFERENT YEARS*. So, with 2015 as the final year (as hinted by the "updated 2015" text), the new order goes:

-2010 Columbus Day
-2011 Christmas
-2012 Flag Day
-2013 April Fool's Day
-2014 Thanksgiving
-2015 New Year's

What do you do now? In all the text, The lEtters weRe wriTten stranGely. This should tell you exactly what to do: take the first letter of the first holiday, second of the second holiday, etc. And when you do this, the answer is spelled out clear as day:

C.H.A.I.K.A

Congrats to those who solved the puzzle~~