Mnemonics, up and down
Going up: Happy Birthday lifts a major 2nd on "hap-py BIRTH-", and Frère Jacques climbs its first two notes by one. Going down: Yesterday sighs down a whole step on its first word, and Mary Had a Little Lamb starts its nursery descent with the same interval.
The truth is you barely need the songs. Sing any major scale and stop after two notes; that's the interval. The mnemonics are there for the moment in a round when your mind goes blank and a tune is faster to summon than a scale.
Jazz keeps its own pair of reference tunes. Mark Levine's interval table fixes the ascending major 2nd to Miles Davis's "Four" and the descending one to his "Tune-Up."
On the staff
The step everything walks on
The whole step is the brick of Western melody. The major scale is five of them and two half steps in a fixed pattern; most tunes move by step most of the time, and most of those steps are major 2nds. That ubiquity is why ear training treats it as a reference point: the question is rarely "was that a major 2nd?" so much as "was that leap bigger than a step?"
Sounded together, the two notes have a mild, even pleasant rub. Theory files both seconds under dissonance, but this one wears it lightly; jazz pianists fold added 2nds and 9ths into chords precisely for that soft friction. Melodically there's no tension at all. It's the most neutral move music has.
What it gets confused with
The minor 2nd below it: the bristling, urgent version of a step. The pair is covered from both sides on these two pages; the short version is Happy Birthday versus Jaws. The minor 3rd above it is the other boundary, and it's a boundary of kind rather than degree: a 2nd sounds like a step in a line, while a 3rd has a small hop in it, the first hint of a chord. When an interval sits right on that edge, ask whether the two notes could be neighbors in a scale.
How Aubel handles it
The major 2nd unlocks third beyond the starting set, after the major 3rd and major 6th. It arrives as the smallest interval in the pool by a wide margin, which makes its early rounds nearly free, and its real work begins later, when the minor 2nd and minor 3rd show up on either side of it. The detail screen leads with Happy Birthday.