Surah Al-Muddaththir 74:9

فَذَٰلِكَ يَوْمَئِذٍ يَوْمٌ عَسِيرٌ

That Day will be a difficult day

Translation: Saheeh International

Tafsir

that day that is to say the time of the sounding yawma’idhin is a substitution for the preceding subject and is not declined because it is annexed to something that cannot be declined; …

Excerpt from Tafsir al-Jalalayn, commentary on 74:9, via Quran.com. The reader has the full commentary and 9 more tafsirs.