Caption: A diagram illustrating nodal alginments which determine the eclipse seasons: i.e., the periods when eclipses of any kind can occur.

Features:

1. Eclipse seasons occur when the lunar node line aligns with the Earth-Sun line. (See also Wikipedia: Orbital node.)

2. Because the Earth, Moon, and Sun have finite sizes, exact alignment of the lunar node line (i.e., an exact nodal alginment) and the Earth, Moon, and Sun are NOT needed for an eclipse. Just close enough alignment.

   So eclipse seasons have a finite time length.

   Whether an eclipse of any kind happens in an eclipse season depends on whether the Moon is in the right place at any time in the eclipse season.

3. Eclipse seasons (counting all types of eclipses) range from 31 to 37 days with mean length 34 days (see Wikipedia: Eclipse season: Details). Since even the shortest length is more than the mean lunar month = 29.530588853 days (J2000), every eclipse season there is at least one lunar eclipse of some type (total lunar eclipse, partial lunar eclipse, penumbral lunar eclipse) and at least one solar eclipse of some type (total solar eclipse, annular solar eclipse, partial solar eclipse, hybrid solar eclipse). The lunar eclipse and solar eclipse can happen in any order. There can even be 3 eclipses: lunar eclipse, solar eclipse, lunar eclipse or vice versa. The case of 3 eclipses happens when one eclipse happens just at the start of an eclipse season: the Moon then has enough time in the lunar month to race around the Earth an pick up 2 more eclipses before the end of the eclipse season. However, yours truly does NOT think that 2 "total" eclipses of any type (i.e., total lunar eclipse, total solar eclipse, and annular eclipse) can ever occur in a single eclipse season. But yours truly CANNOT at this moment find a reference that says this explicitly.

   Also, yours truly believes eclipses that happen just at the start or end of an eclipse season can NEVER be "total" eclipses But again yours truly CANNOT at this moment find a reference that says this explicitly.

4. Note that the eclipse season for total lunar eclipse and partial lunar eclipses (i.e., excluding penumbral lunar eclipses) is only 24 days (from 12 days before exact nodal alginment to 12 days after) (Mo-128). So you can have eclipse season with NO total lunar eclipse and NO partial lunar eclipse. Most people consider such an eclipse season a dud for lunar eclipses, since penumbral lunar eclipses are almost unnoticeable and no one pays much attention when they occur.

   In fact, most people probably consider an eclipse season a dud for lunar eclipses if there is NO total lunar eclipse. Since total lunar eclipses actually happen in only about 28.7 % of eclipse seasons (see Table: Frequency of Lunar Eclipse Types for 3000 BCE--3000 CE at Eclipse Seasons (AKA Nodal Alignments) ), most eclipse seasons are probably duds for lunar eclipses for most people.

   Note also if two solar eclipses happen in an eclipse season, the lunar eclipse between them is always a total lunar eclipse (Mo-128).

5. Counterfactually, if the lunar node line had NO absolute rotation (i.e., rotation relative observable universe: i.e., to the bulk mass-energy of observable universe) (see Wikipedia: Inertial frame of reference: General relativity), nodal alginment would happen every 6 months or a bit more precisely about every 182.6 days or even more precisely every half sidereal year = 365.256363004 days (J2000).

6. However, the lunar node line does rotate relative to the observable universe. The relevant J2000 values are:
   1. rotation rate westward 19.355331540 degrees per Julian year (Jyr). Note westward means counterclockwise looking down from the north celestial pole (NCP).
   2. rotation period 18.599526402 Julian years (Jyr).
   3. Time between eclipse seasons (i.e., exact nodal alginment) 173.310037942 days ≅ 173.31 days. Note a day here is the standard metric day = 24 h = 86400 s.

   See also Wikipedia: Eclipse season: Details.

   Now I know what you are thinking: why oh why must the lunar node line rotate?

   For an exact gravitational two-body system of just Earth and Moon isolated in space, it would NOT rotate. But the Earth-Moon system is surrounded by other Solar System objects which exert gravitational perturbations on the Earth-Moon system.

   It's enough here to say that gravitational perturbations cause the rotation of the lunar node line.

7. Because of the 173.31 days between nodal alginments, the eclipse season eventually cycles through the entire calendar year. Thus, eclipses can occur at any time of the calendar year.

   Usually there are just 2 eclipse seasons. But since 173.31 days is less than half a year (of any kind), there occasionally can be 3 eclipse seasons in a year: one near the beginning, one near the middle, and one near the end.