Sort the months of the year
Jelly, 19 bytes

11ị“bMAanlseovc”iµÞ


This is a monadic link that takes a list as argument and sorts it. Try it online!

Background

Jelly uses modular, 1-based indexing. If we repeat the months names often enough to obtain 11 characters, we get the following array.

J a n u a r y J a n u
F e b r u a r y F e b
M a r c h M a r c h M
A p r i l A p r i l A
M a y M a y M a y M a
J u n e J u n e J u n
J u l y J u l y J u l
A u g u s t A u g u s
S e p t e m b e r S e
O c t o b e r O c t o
N o v e m b e r N o v
D e c e m b e r D e c


In the 11th (last) column, all characters are different, so we can use them to identify the order of the months.

How it works

11ị“bMAanlseovc”iµÞ  Monadic link. Argument: A (array of months)

                 µ   Combine the preceding chain into a link.
                  Þ  Sort A by that link.
11ị                    Select the 11th character of the month's name.
   “bMAanlseovc”       Find the index of that character in "bMAanlseovc".
                       For 'u' ("January"), this returns 0 (not found).