Jamieson, Fausset and Brown on Jeremiah 51:41

41. Sheshach--Babylon (compare Note, see Jr 25:26); called so from the goddess Shach, to whom a five days' festival was kept, during which, as in the Roman Saturnalia, the most unbridled licentiousness was permitted; slaves ruled their masters, and in every house one called Zogan, arrayed in a royal garment, was chosen to rule all the rest. He calls Babylon "Sheshach," to imply that it was during this feast the city was taken [Scaliger].