Example 2. What day of the week was January 1, 2000?
$$
\begin{array}{c}
\text { Sol } C=19, Y=99 . m=11 . d=1 \text {. } \\
W \equiv 1+28+1+3+4-38(\bmod 7) \\
\equiv 6(\bmod 7) .
\end{array}
$$

So the first day of the next century is Saturday.
$$