Example 1. What day of the week was October 1, 1949?
$$
\begin{array}{l}
\text { Solve } C=19, Y=49, m=8, d=1 \\
W \equiv 1+20+49+12+4-38(\bmod 7) \\
\equiv 6(\bmod 7) .
\end{array}
$$

That is, October 1, 1949 is a Saturday.