There have been seasonal changes before. In Christmas 2011 (the show's first year), a new segment was inserted, and for Valentine's Day 2012, a love-theme section was added. The romance addition has stayed there, actually, and is still there during the summer, even side by side with the additions.
The summer additions start with a brief introduction by Phineas and Ferb (who famously try to find ways to fill 104 days of summer vacation), and we are treated to the sand castle, Crush from Finding Nemo, Goofy surfing, Timon and Pumbaa from the Lion King, and Ariel from Little Mermaid.
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Kevin Yee is the author of numerous independent Disney books, including the popular Walt Disney World Earbook series and Walt Disney World Hidden History.