Harmonix gave every character a birthday. You can find it out in one of two ways. You can notice that you got an extra star for playing a song with the character, or you can consult the list of birthdays HERE. After you know when the character's birthday is (which is usually announced on Twitter or Facebook, too), go into perfrom, pick the song "In Da Club," and select that character. If you cannot wait, you can change your console's date to the character's birthday and play offline. Rasa (the first character you use in the story) has his birthday on October 16th (Launch day for the game in North America) if you do not feel like consulting the list and are going to change your date settings.