CLAMP said on their Diary during Tsubasa's inital serialization that there are 3 patterns for the title page of each chapter:

• Illustration (has nothing to do with the story)
• Past (the scene on the past)
• Future (a preview for scene on the future)

April 1st is a special day for CLAMP. It is the founding of their group as well as the birthday of Sakura and Watanuki.

According to the ICv2 Retailers Guide to Anime/Manga #15, Tsubasa is the 6th most popular manga property in the United States, beating out titles such as Death Note, Bleach, and Rurouni Kenshin. [Source]