Cristiano Ronaldo joined Manchester United in the 2003-04 season, after signing in August 2003. His move came before the start of that campaign, and he made his debut in Manchester United’s season opener against Bolton on 16 August 2003.
That first spell at United ran through the 2008-09 season, before he left for Real Madrid in 2009.
The season in context
Ronaldo arrived as a teenager from Sporting Lisbon and quickly became one of the club’s most important players. By the end of his first stint, he had developed from a raw winger into a world-class forward.
If you are asking about his second return, he rejoined Manchester United in the 2021-22 season, after a transfer agreement was reached in August 2021.