I Know What We're Gonna Do in May was a multi-part marathon featuring most the Phineas and Ferb television series, which was promoted as airing during the entire month of May 2018 on Disney XD. Beginning on Tuesday, May 1, 2018, the daily promos for the marathon stated that it started at 5pm each weekday, when Disney XD aired a three hour block of several episodes.[1] (At least one evening had a block of three and a half hours, with the last episode of the night being "Night of the Living Pharmacists".) The last such evening block and promos for it aired on Thursday, May 24, 2018. The next day saw the beginning of the Phineas and Ferb Takeover Takedown Marathon, though whether that was officially a separate marathon from I Know What We're Gonna Do In May was not made officially clear. (Both marathons had promos featuring scenes from Take Two with Phineas and Ferb, though the Takeover Takedown marathon only seemed to air a maximum of six episodes of that show.) Separate or not, the Takeover Takedown marathon was over by May 28, 2018, so at least one promo's claims of I Know What We're Gonna Do In May spanning "all May long" had fallen short by at least the last three days.[1]
There was no indication that this marathon included Phineas and Ferb episodes that Disney XD was already airing on weekday mornings and afternoons, nor any that aired on the weekends before May 25, 2018.