On another StackExchange QA site, I asked a math question involving Friday 13th. It deals with years having more than one Friday 13th.
I used 'Fridays 13th' as the plural form of 'Friday 13th'. Was I right? Or it should be 'Friday 13ths'? Or even 'Fridays 13ths' (that somehow looks nice to me, but I doubt it is correct)?
- Google search for +"fridays the 13th" gives 36.600 results;
- Google search for +"friday the 13ths" gives 239.000 results.
Examples of reputable internet sites using both versions: