What is the forbidden slander? And is it permissible to say: "This day is disgusting"?

The questioner seems to think this refers to reviling time (ad-dahr), and the difference between reviling time, describing it, and reporting about it has already been clarified. There is no harm in describing time or reporting about it, such as in the Almighty’s saying: "So We sent upon them a furious wind during days of misfortune." As for reviling (sabb), it is any foul speech, and its rulings vary: most of its rulings are prohibition (hurmah), and it may reach the level of disbelief (kufr) if it involves reviling Allah, the Messenger, or the angels. It may also be disliked (makrooh), like reviling a fever. It may also be contrary to what is better (khilaf (definition) al-awla), or it may be permissible (ja'iz), like reviling wicked people or reviling someone who reviled you, to the extent that they reviled you.