The invitation is not used anywhere. The pen is used at the football scene with Lucy and Charlie Brown. But there are two scenes between getting the pen and using it: Snoopy gathering leaves and Linus losing his blanket in them.
Go left and find Charlie Brown and Snoopy to start the leaf-blowing game.
When you give him the lemon sucker from Pigpen. He gives you his invitation to the party, and you can get his pen from the desk.
It is on the table after you give Linus the lemon sucker from Pigpen. Linus gives you his invitation to the Halloween Party, and he has just written a letter to the Great Pumpkin. Pick up the pen.
You get the invitation from Linus, who is not planning to attend anyway. When you return the pumpkin for him, he gives you a trick or treat bag, which you give to Pigpen (outside), who gives you a lemon sucker. Take this back to Linus and get his invitation. Also, pick up his pen.
Pick up his pen, and go outside, to the far left where Charlie Brown and Snoopy are gathering leaves.
After you get the invitation and the pen, Charlie Brown and Snoopy are in his backyard, on the far left of the island. You will play a mini-game as Snoopy gathers some leaves in a very relaxed manner.
You can find the pen after you give Linus's candy bag to Pigpen, who is on the street. In return, he will give you a lemon-flavored sucker. Then go back into the house and give it to Linus, who gives you his party invitation. Pick up the pen, which he was using to write a letter to the Great Pumpkin.
After you take the bag to Pigpen, and bring back the lemon sucker for Linus, he gives you his invitation. His pen is there on the desk, and you will need it for the football scene later.
Once you have the invitation and the pen, go outside, far left, to where Snoopy and Charlie Brown are gathering leaves.
You can get a mask, a sucker, an invitation, a pen, a blanket, and a football. P.S. You have to use most of those for the game tho so you would only keep the mask and the invitation in the end.
Go back into the house and give it to Linus, as it is his favorite flavor. He will give you his party invitation. You should also get his pen from the desk.
The pen, which you need to do the Charlie Brown football scene, is on Linus's desk in Linus and Lucy's house. It will appear when Linus is writing his letter to the Great Pumpkin. This is after you have selected the pumpkin in the pumpkin patch and rolled it back to the house. Take the bag that Linus gives you and give it to Pigpen on the sidewalk outside. He will give you a lemon sucker. When you take this back inside and give it to Linus, he gives you his invitation to the Halloween party, and the pen is on his desk. Linus is writing a letter to the Great Pumpkin.
You get it from Linus by giving him the sucker.