Caption: A schematic diagram of the Moon's orbit and the lunar node line.

Features:

1. Moon's orbit has an inclination to ecliptic of 5.145%deg; (see Wikipedia: Moon: Table). Thus the Moon's orbit crossed the ecliptic plane in only two places called lunar nodes which are the Moon-specific name for orbital nodes

   The line that connects the nodes (and which passes necessarily through the Earth) is called---very imaginatively---the lunar node line.

2. The lunar node line rotates westward 19.3549 degrees per Julian year (Jyr) (see Wikipedia: Orbit of the Moon: Inclination, J2000 epoch???). relative to the local inertial frame of the fixed stars (effectively that of the Solar-system barycenter).

3. Now I know what you are thinking.

   Why, why must the lunar node line ROTATE?

   In an exact gravitational two-body system, the lunar node line would NOT rotate relative to the inertial frame.

   But the Earth-Moon system is a two-body system only to 1st order So the orbits of Earth and Moon about their mutual center of mass (which is origin for their free-fall frame: i.e., local approximate inertial frame) are simple only to 1st order

   The Sun and to a much lesser degree the planets add complicated gravitational perturbations to the Earth-Moon system. This results in subtler, complex motions.

   We will, of course, not go into the celestial dynamics of those subtle motions. But there seems to be an endless regression of them. Once you've detected and analyzed one, there is another smaller, more subtle one to deal with. Yours truly's patience quickly runs out.

4. Eclipses can only happen when the lunar node line is approximately aligned with the Earth-Sun line. The exact alignment occurs every 173.31 days and the time frame around exact alignment when eclipses can happen is 31 to 37 days (see Wikipedia: Eclipse season: Details).

   This time frame is called the eclipse season or, as yours truly often says, a nodal algnment---it trips off the tongue.

5. Why every 173.31 days?

6. If counterfactually the lunar node line was fixed relative to the fixed stars, exact alignment would occur twice per sidereal year (rather precisely 365.256363004 days on 2000 January 1 noon, AKA J2000): i.e., would occur approximately every 182.63 days.

7. However, aforesaid westward rotation of the lunar node line reduces the time between nodal algnments to the aforesaid 173.31 days (see Wikipedia: Eclipse season: Details).

8. Because of the rotation of the lunar node line, the eclipse season eventually cycles through the whole calendar year, and so eclipses can happen at any time in the calendar year.