The plural of "Christmas" is "Christmasses", surely?

[MadreInglese]

Just seen a poster for a new film called "Four Christmases"

This looks wrong, shouldn't there be an extra "s"?

It does look funny as we don't usually see christmas written as a plural. But it is indeed correct. It does not need another 's' as the 'es' does the job.
Hope that helps .

It looks odd

Should it not be 'Christmas's'?

I think Christmases is right.

I just asked google this question an this was the first link [santa]

DaisyMooSteiner is right about the plural being Christmases. It confuses a lot of people. If you look at Wikipedia and this page, plural of Christmas, both refer to the plural as Christmases.