This is the 17431st most frequent Polish word.
zasiąść
"Zasiąść" means "to sit down" or "to take a seat" (often with a sense of formality or purpose).
Here, 'zasiadł' means to take a seat in a formal and significant manner, typically related to positions of authority.
Zasiadł na tronie, aby ogłosić swoje decyzje.
He sat on the throne to announce his decisions.
In this context, 'zasiądą' refers to the gathering of people taking their seats to commence a communal activity.
Wszyscy zasiądą przy stole wigilijnym na kolację.
Everyone will sit at the Christmas Eve table for dinner.
Here, 'zasiadła' indicates sitting down in a calm and intentional way to relax.