The period before 1st period.
1st period being the first "official" period of the day, which now can't begin before 8:30a.
Originally, it began life as the before-school time for athletics and other organized extra-curriculars, but once it existed, it became a full-fledged period of instruction, as high-achieving students needed to cram ever more "elective" courses in order to compete for the top university slots.
Even 25 years ago when I was in high school, there was talk of creating a double-zero for the athletics, the marching band, and the extra-curriculars, so that elite students could "still afford" to do them.
In my district, it didn't happen (With zero starting at 7:08a, double-zero would have been 6:03a, if I'm remembering class length correctly). One of the reasons why it didn't happen is that since there is no "honors athletics" which can be graded on the 5-point scale, elite students couldn't afford the grade points to take them anyway.