Parva days are mentioned in Manu Smriti on multiple occasions. Here is one such instance:
4.150. Let him always offer on the Parva-days oblations to Savitri and such as avert evil omens, and on the Ashtakas and Anvashtakas let him
constantly worship the manes.
A different translation of the above verse and commentary on it can be found from here:
Indra is a position and occupied by different people at various times. Please see Narada purana for the list of manvantaras and the indra of each manvantara.
“I will first tell you about the manvantaras,” replied Sudharma.
There are fourteen such eras in one of Brahma’s days. Each such era is
ruled over by a Manu. And the gods also change from one ...
The story, described in the Bhavishya Parva of the Harivamsa, is related to the aftermath of Janamejaya’s sarpa satra and him having finished listening to Vaishampayana’s narration of Mahabharata and Shri Krishna’s Leela. Janamejaya performed the horse sacrifice where Indra possessed the horse and having intercourse with the queen (actually Apsara Rambha), ...