"Grampu Day!" is an episode of Oobi from the first season.
Grampu bakes cookies for Oobi, Uma, and Kako. As the kids eat, Oobi pretends to be Grampu. Uma and Kako show off their Grampu impressions as well. They all talk about Grampu and how friendly he is. This inspires them to create a holiday in Grampu's honor called "Grampu Day". To celebrate, they decide to give him a statue, special food, and a special present. When Grampu comes back, they wish him a happy Grampu Day, and tell him the plan.
Oobi goes off to make a clay statue, with Grampu serving as a model. While Oobi is sculpting, Grampu develops an itch, and he tells Grampu he can scratch as he finishes the model. To Oobi's disappointment, it looks nothing like Grampu. Trying to spare Oobi's feelings, Grampu says he thinks it does. He awkwardly tries to move into the position of the statue.
Uma walks into her room in search of a present, but all she finds is dirt. Meanwhile, Kako is in the kitchen making Grampu some special food. He thinks about the foods Grampu likes: tuna, which Grampu likes, and jelly, which Grampu loves. He mixes the two things together. He tastes it, but finds something's missing, so he adds hot sauce, which he loves. He gives the food to Grampu, who is excited to try it. However, one bite makes him gasp for air. Grampu has trouble swallowing it. Grampu tries not to hurt Kako’s feelings either by pretending to enjoy it and saying "Mmm". Kako then asks the viewers to do the same as he continues to feed Grampu the disgusting dish.
Uma is still searching for a present. She enters the bathroom, but can only find a used tube of toothpaste. The children gather in the living room to celebrate, where Uma tells them that she has had no luck finding a present. Oobi gets an idea and whispers it to Uma. She agrees with his plan and sings a special song to Grampu. The children hug Grampu, who thanks them.
Later, Oobi and Kako find a pile of leftover clay. Oobi gets the idea to play a guess game with it. He creates a statue and asks the viewers to try and guess what it is, which is revealed to be Kako. Afterwards, Oobi and Grampu ask the viewers about their grandparents. A group of preschoolers are shown talking about their families. The episode ends.