"I am busy on Monday."
I made a comment about this here:
using に with states like i-adjectives or with a verb like ある for example, describe the ends/objective of that state, here is a more clear example「試験の準備に忙しい」"I'm busy preparing for the test" or "I'm busy with the preparation for the test". You can also find に in adverbial expressions like「常に忙しいです」"I'm always busy".
I'm sure you can find examples with に(i-adjective), but in those cases, it will almost always describe the state rather than pointing to a point in time. 私は月曜日忙しいです and 月曜日は私が忙しいです should both be correct though.
This is something I picked up by osmosis, not sure about the grammar rule behind, so if anyone knows the why, let me know.