I tried putting the struct definition in an .h file and #including the .h file, I swear! 
But that created a different problem. Then I got
error: 'byte' does not name a type for each member of the structure (and yes, I #included <stdint.h> and <stdbool.h>)
I found a simple example on StackOverflow: Arduino: struct pointer as function parameter - Stack Overflow
My struct definition was fine, and I was able to leave it in my main .ino file. But as pylon mentioned above, I needed to put "struct dateTime" instead of just "dateTime" in my function declaration. I also needed to use a pointer, and not by reference (or value)
So my function became:int isHoliday(struct dateTime *dt, int holidays[]) and I called it with:holidayCount = isHoliday(&curDT, holidayList); sending it by reference.
Then in my isHoliday function, I had to access the members of dateTime using "->" instead of "."
My remaining problem of not being able to initialize the members, I solved by accident by moving that code to within the setup() procedure instead of before it, at the top.
Thanks for everyone's help!