Year 2001 was a common year starting on Monday. This year began the third season, aired the first and second half of the second season, and concluded the first season. This is the first year of SpongeBob to have a 4-month hiatus, which lasted from May 19 to September 7. "Shanghaied" aired on March 9, 2001, but its partner episode, "Gary Takes a Bath" didn't air for another 2 years until July 26, 2003.
An episode from all three seasons aired this year. The next time that three seasons aired episodes within the same year would happen in 2009, with seasons 5, 6 and 7. And also 2017, with seasons 9, 10, and 11.