Question: On 8th Dec, 2007, Saturday falls. What day of the week was it on 8th Dec, 2006?

Answer:
To find out the day of the week on a given date, we can make use of a specific method called the Zeller's Congruence. This method allows us to calculate the day of the week for any given date in the Gregorian calendar. Let's apply this method to determine the day of the week on 8th Dec, 2006.

Zeller's Congruence:
Zeller's Congruence is an algorithm devised by Christian Zeller to calculate the day of the week for any given date. The formula is as follows:

\[ h = (q + \frac{13(m+1)}{5} + K + \frac{K}{4} + \frac{J}{4} + 5J) \% 7 \]
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 mod 100)
- J is the zero-based century (actually ⌊ year/100 ⌋)

In our case:
- q = 8 (day of the month)
- m = 10 (December is considered as month 14 of the previous year)
- K = 6 (2006 mod 100)
- J = 20 (⌊ 2006/100 ⌋)

Calculation:
Substituting the values in the formula, we get:
\[ h = (8 + \frac{13(14+1)}{5} + 6 + \frac{6}{4} + \frac{20}{4} + 5(20)) \% 7 \]

Simplifying the equation step by step:
\[ h = (8 + \frac{13 \times 15}{5} + 6 + \frac{6}{4} + \frac{20}{4} + 100) \% 7 \]
\[ h = (8 + 39 + 6 + 1 + 5 + 100) \% 7 \]
\[ h = 159 \% 7 \]
\[ h = 3 \]

Therefore, the day of the week on 8th Dec, 2006 was Friday (where 0 = Saturday, 1 = Sunday, 2 = Monday, ..., 6 = Friday).