Belly's birthday in "The Summer I Turned Pretty" TV series is celebrated at the end of June, with strong indications pointing to June 20 as the exact date. This is a change from the original books by Jenny Han, where Belly's birthday is in August, marking the end of summer. In the TV adaptation, the birthday occurs earlier in the summer, around June 20, as revealed through subtle clues like the placement of an infinity necklace on that date in a calendar during the show. Throughout the series, Belly's birthday serves as a significant plot point marking stages of her growth and relationships, including her 16th and 21st birthdays and culminating in an important 22nd birthday in the final season.