The story goes that the Sun and the Moon sat next to the demon Svarbhanu in the elixir line.
After the churning of the ocean, the elixir or immortality was partaken of by all the gods. Svarbhanu slipped into the elixir food line and swallowed their elixir before any of the gods could do anything about it.
The Sun and Moon who were sitting next to Svarbhanu raised the alarm. Mohini/Sri Vishnu hit him with her ladle, with which she was serving the elixir, and cut him in half but since he/they had already taken the elixir, no good came of the slicing: Svarbhanu became two — Rahu and Ketu, body and head of a serpent. The blood drops that fell when Svarbhanu transformed into Rahu and Ketu is said to have turned into garlic and onion, which is why garlic and onion are considered non vegetarian by strict Hindu households.
Chhat Puja is the culmination of the offering of Arghya to the Sun and the Moon, the setting sun (and the rising moon) in order to fortify them and strengthen them because they have to forever fight the immortal Rahu and Ketu.
Offerings are made to the sun goddess by women and men alike. Chhat is gender neutral that way.
One big difference you’ll notice in the setting sun pictures versus the rising sun pictures is that everyone faces west for the one and east for the other. I’ve only attended the setting sun arghya so far and for all my knowledge of geography and the solar system, I was taken aback to find everyone pointing in a different direction “this year.”
Of course, they were just following the sun. Last evening, they were all facing west like I’m used to.
I’m leaving you with pictures of my silk-swathed self, orange anointments in place for the last prayers. Wish you all a very happy chhat puja with a happy and prosperous coming year.