A 2026 graduate of Dana Hills High School, the Laguna Niguel, Calif. native earned four letters in volleyball, two in beach volleyball and two in softball. Playing for a Division 3 CIF Southern Section program under head coach Tom Hoff, Day helped Dana Hills capture the South Coast League championship in 2024 and finish as league runner-up in 2025; that season, the team also advanced to the semifinals of the state championship. The 5-foot-9 outside hitter posted a career-high 16 kills in a match against Edison High School as a senior and earned Academic Excellence and Superintendent's Honor recognition throughout her four years at Dana Hills. She was also named beach volleyball MVP as a senior and spent two seasons on the softball team.

Chelsea is the daughter of Stephen and Kristie Day and plans to major in biology.