I'm not sure whether it was intended by CLAMP or not, but... have you ever noticed that some CCS characters' birthdays are on special days or holidays!? As far as special days go, Sakura's birthday occurs during the short period in which cherry blossoms bloom in Japan, and Touya's birthday is on February 29 which only occurs every 4 years! Regarding holidays, Yukito's birthday falls on Christmas Day, and Mizuki-sensei's birthday occurs on National Foundation Day.

The table below displays American federal holidays, Japanese national holidays, and special days. One bank holiday has been included as well.

The date in which these holidays fall varies from year to year.
^A An American federal holiday.
^J A Japanese national holiday.
^S A special day.

Note: Isn't it also interesting that Sonomi and Nakuru share the same birthday!?