The day of the week on 21st September 1987 was Monday.
Explanation:
- **Finding the day of the week**: To determine the day of the week for a specific date like 21st September 1987, we can use the Zeller's Congruence formula. This formula calculates the day of the week for any given date in the Gregorian calendar.
- **Zeller's Congruence formula**: The formula is as follows:
\[ h = \left(q + \left\lfloor\frac{13(m+1)}{5}\right\rfloor + K + \left\lfloor\frac{K}{4}\right\rfloor + \left\lfloor\frac{J}{4}\right\rfloor - 2J\right) \mod 7 \]
where:
- h is the day of the week (0 = Saturday, 1 = Sunday, 2 = Monday, ..., 6 = Friday)
- q is the day of the month
- m is the month (3 = March, 4 = April, ..., 12 = December; January and February are counted as months 13 and 14 of the previous year)
- K is the year of the century (year % 100)
- J is the zero-based century (year / 100)
- **Plugging in the values**:
- For 21st September 1987, q = 21, m = 9, K = 87, J = 19 (as 1987 belongs to the 20th century)
- Calculating the value of h using the formula will give us the day of the week, which is Monday in this case.
Therefore, the day of the week on 21st September 1987 was Monday.