If Thursday will fall on 5th of a month, then which day will fall three days after the 20th of that month?

1. Given that the 5th of the month is a Thursday.
2. "Three days after the 20th" means the date is:

$$20+3=23\text{rd}$$

3. Let's find the day on the 23rd by adding multiples of 7 days to the 5th:

* $5+7=12\text{th}$ is Thursday
* $12+7=19\text{th}$ is Thursday

4. Now, count forward from the 19th to the 23rd:
* 20th = Friday
* 21st = Saturday
* 22nd = Sunday
* 23rd = Monday

Therefore, the day will be Monday.