فِي يَوْمٍ

فِي يَوْمٍ is a phrase (شِبْهُ الجُمْلَةِ) consisting of a particle of pulling down (preposition) and a genitive noun

PhraseParticle / NounNoun Particle
شِبْهُ الجُمْلَةِجَارٌ وَمَجْرُوْرٌاسْمٌ مَجْرُوْرٌحَرْفُ جَرٍّ
PhraseCompoundGenitive NounParticle of pulling down
  
on a dayفِي يَوْمٍ=يَوْمٍ+فِي
in a daya dayin
Surah/Below are the 4 verses in the holy Quran containing the preposition and genitive noun فِي يَوْمٍ
Verse
14-18
32-5
70-4
90-14or giving food in a day of hungerأَوْ إِطْعَامٌ فِي يَوْمٍ ذِي مَسْغَبَةٍ