What was the day of the week on 15 Aug 2019, given that 1st Jan 2025 is a Wednesday?

Let's figure out the number of days between 15 Aug, 2019, and Jan 1st, 2025.

Let's find the number of days from 15 Aug 2019 to Jan 1st 2020.

Aug - 16, Sept - 30, Oct - 31, Nov - 30, Dec - 31. Their sum is 138. 

2020 - 366

2021 - 365

2022 - 365

2023 - 365

2024 - 366

2025 - 1

The sum of these days = 1828.

Total 138+1828 = 1966 odd days.

1966% 7 = 6.

Six days back to Wednesday will be Thursday

So, the day is Thursday.