January shortens to Jen – how to correct?

Hello!

Your plugin works nicely on our page: https://www.overtonphotographicclub.co.uk/programme/

However, if you scroll to January, you’ll see that in the black boxes ‘Jen’ appears when ‘Jan’ would be correct in the English language.

How can I correct this so that the short version reads ‘Jan’?

Many thanks

The page I need help with: [log in to see the link]