Is "We have a test in Monday" correct?

Why

Priya has correctly noticed that "in" is used for time — "in the morning", "in June", "in 2024" all genuinely use "in". Where the rule needs a second branch is realising that English splits time expressions across three different prepositions depending on how BIG the time period is, not just one preposition for everything. "In" covers longer stretches — months, years, seasons, parts of the day. "On" covers specific days and dates — "on Monday", "on my birthday", "on 15th August". "At" covers specific points in time — "at 5 o'clock", "at noon". A single day like Monday is neither a long stretch nor an exact point, it's its own category with its own preposition. Test: notice you'd never say "in my birthday" or "at Monday" — each size of time slot has learned its own matching word, the way each key fits only its own lock. Her brother is right: specific days take "on", so it's "on Monday", not "in Monday".