We now know the first 6 quests for A Ripple in Time. Once the last two become available, I will update this list. Good luck in your search for Celebi!
Update: We now know all 8 quests to catch Celebi! It appears that there is really only 7 quests to be done. Then, after catching Celebi, the final quest is automatically completed with more rewards and additional dialog from Professor Willow.