That's where Nicole Westbrook came in.
Or rather, that's where Patrice Wilson came in. He wrote "It's Thanksgiving" in probably as much time as it took him to write "Friday" for Rebecca Black, and sent it off to be performed by Westbrook. The rest is history.
Also like "Friday," it displays a curious focus on the passage of time. Where "Friday" counted down the days of the week in anticipation of the titular one, "It's Thanksgiving" also ticks off several major holidays (Christmas, New Year's Day, Easter, and the Fourth of July) to build to Thanksgiving, though it's anyone's guess as to why Valentine's Day or Halloween – to say nothing of Arbor Day – were excluded.
Fingers crossed for a Patrice Wilson "I Love You, Arbor Day" video.