> What if I
>offered the option for either a letter each month, or just a
>letter per holiday, including their birthdays.


It's always great to offer choices, so I would advise offering both the monthly subscription and the holiday/birthday. This way parents and grandparents can choose which way to go. I'd bet you get many that want BOTH. Or even offer the birthday letter separately. That way the monthly subscribers could buy the birthday letter too, but may not want all the holiday letters.

I think you're going to have a big hit with this!