Know some history before doing math

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Zeller's congruence is an algorithm devised by Julius Christian Johannes Zeller to calculate the day of the week for any Julian or Gregorian calendar date. For the Gregorian Calendar, the formula is

\[w=\bigg ( y +\bigg \lfloor \frac{y}{4} \bigg\rfloor + \bigg\lfloor \frac{c}{4} \bigg\rfloor -2c + \bigg\lfloor \frac{26(m+1)}{10}\bigg\rfloor+ d-1\bigg) \mod 7\]

What is the earliest date when the formula could be applied to calculate the day of the week?

Hint : Gregorian Calendar


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