Answer:
Thursday
Explanation:
Since one week is equivalent to 7 days, lets find how to write 2019 days as a combination of full weeks (which will land one again on Monday), plus days:
Notice that 2019 = 2016 + 3 = 288 * 7 + 3
Then 2019 days is the same as 288 weeks plus 3 days.
The 288 weeks from Monday will take us to another Monday, and the added 3 days to a Thursday.