The length of our day is 24 hours. Yes, but that is only its mean length, the average time between noons, the Sun’s crossings of the meridian. If you time those crossings with a stopwatch, you find that the Sun sometimes reaches the meridian slightly early, sometimes slightly late. So the observed, or “apparent”, lengths of the day varies.
This is partly because the Earth travels slightly faster in the inner half of its slightly elliptical orbit (centered on perihelion, around Jan. 3) than in the outer half; but rotates at constant speed.
From perihelion (early January) onward a spot on Earth begins arriving slightly later each day at the direction facing the Sun, because Earth is slowing in its orbit. (Exaggerated in this diagram.)
But the larger reason is Earth’s obliquity: the tilting of its north pole 23.4° away from the Sun around the December solstice and toward it around the June solstice. Around the times of the December and June solstices (from early November to early February, and from early May to early August), the Sun travels across the lines of right ascension on the map of the sky faster than it is traveling in longitude; around the March equinox (from early February to early May) its course slopes north in right ascension, and around the September equinox (from early August to early November) it slopes south, so at those times the Sun’s right ascension changes more slowly than its longitude. Another way of putting it is that, as the Sun moves one day’s journey along the ecliptic, it moves more or less than that angular distance in right ascension, which is the measure of when it is on the meridian at noon.
The difference between apparent solar time (the actual time between noons) and mean solar time (the average time between noons) is called “the equation of time.” This old use of equation means that by subtracting it from the varying apparent time you equalize that to the average.
The two factors combine to give a curve with two minima and two maxima.
The dates can vary slightly from year to year because of leap days. These are the dates for 2020. The equation of time
Feb 11 is at minimum for the year, -14.24 minutes.
Apr 15 3 is 0.
May 13 reaches a shallow maximum of 3.65 minutes.
Jun 12 is 0.
Jul 25 reaches a shallow minimum of -6.55 minutes.
Sep 1 is 0.
Nov 2 is at maximum for the year, 16.49 minutes.
Dec 24 is 0.
In February the “equation” reaches its deepest minimum, about 14 minutes. At 12 noon in the regular time told by clocks, the Sun is 14 minutes short of reaching the meridian. The Sun is “slow”, or clocks are “fast” (though this shouldn’t be confused with the speed of Earth in its orbit).
There comes a small maximum in May (at mean noon the Sun is 4 minutes past the meridian); a smaller minimum in July (Sun 7 minutes late reaching the meridian); and a year’s maximum in November (Sun 16.5 minutes early crossing the meridian). In between are dates (in April, June, September, December) when the equation is zero: the midday Sun is just on time!
Here is the graph with the addition of curves for the two components: obliquity (red) and ellipticity (green):
“You can see why the February minimum and November maximum are extreme, while the May maximum and July minimum are moderate, and also how the equation of time is mostly due to obliquity, with eccentricity pulling it first one way then the other.” (Those are the words of friend Anthony Barreiro. He suggested the addition of the curves for the two components, and I can’t improve on his explanation.)