Is pessimism regarding the days of the week considered polytheism (shirk), and is it permissible to pray behind an imam who is pessimistic about them?

Pessimism (tashā'um) and omens (taṭayyur) are shirk (polytheism) and demonstrate a poor opinion of Allah. Therefore, whoever believes that they bring benefit or ward off harm has committed shirk. Whoever finds this in themselves should rely on Allah, trust in Him, and say: "O Allah, there is no good but Your good, no omen but Your omen, and no god besides You."

For the Prophet (peace be upon him) said: "Omens are shirk... There is none among us but [has it occur to his mind]... but Allah removes it with reliance [on Him]."

If the Imam believes that something other than Allah benefits or harms, and acts upon his pessimism, then this is manifest shirk, and it is not permissible to pray behind such a person, due to the saying of the Prophet (peace be upon him): "Whoever is turned away from his need by an omen has committed shirk," and "An omen is only what makes you proceed or turns you back."

However, if it is merely a fleeting thought that is not acted upon, it does not cause harm and is not shirk. This is what Ibn Mas'ud meant when he said: "There is none among us but [has it occur to his mind]," meaning that omens may come to the heart, but Allah removes them through reliance [on Him].