I saw Today's Birthdays.
And I saw Calendar too.
I could see some names that I couldn't see in Today's Birthdays.
Why?

The people listed in Today's Birthdays have all been active in the last week. The people who are on the calender but aren't listed haven't been around for at least 7 months. I'm guessing that somewhere between a week and 7 months there's a cut-off point for who's active enough to list, but I don't know exactly where it is.

Thanks for reply.
I posted "Happy Birthday" in Visitor Messages of all user that appeared in calendar.