Why did Thetis wait 12 days before she made her appeal to Zeus?

Thetis waited 12 days before appealing to Zeus because she was grieving the death of her son, Achilles, and needed time to process her emotions. Additionally, she wanted to ensure that her plea would be impactful, as the situation with the Greeks and Trojans escalated. By delaying her request, she could prepare for a more persuasive argument to persuade Zeus to intervene on Achilles' behalf. This period also allowed her to witness the consequences of Achilles' absence in battle, reinforcing the urgency of her appeal.