«أَنْتِ طَالِقٌ فِي مَجِيءِ يَوْمٍ»
“You are divorced when a day arrives.”
«أَنْتِ طَالِقٌ فِي مُضِيِّ يَوْمٍ»
“You are divorced after a day has passed.”
Arrival of a day concerns the onset of the period; passage of a day concerns completion of its duration. These conditions do not mature at the same instant.
The first asks whether the named time has arrived, while the second asks whether it has elapsed. Sharing the word “day” does not make their legal effects identical.