The scene immediately after Linus gives you the invitation is Charlie Brown and Snoopy in the back yard (left). Snoopy blows some leaves into the pile as they fall, then Linus jumps on the pile. Follow him to the mailbox, then return to get his blanket from the pile.
you go to where snoopy is and then go to the big pile of leaves.Click on it and move all the leaves to the sides.