11. Suppose in a certain year, there is a month where three Sundays fall on odd dates, then what day of the week could the 20th of this month be?



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【Analysis】There are three Sundays with odd dates, and these three Sundays should not be consecutive. Moreover, there should be 14 days between two odd-numbered Sundays. Therefore, we can assume the first Sunday is $x$, then the second Sunday is $x+14$, and the third Sunday is $x+28$. The date of the third Sunday should not exceed 31.

【Solution】Since the interval between each Sunday is 7 days, if a month has three Sundays with odd dates, these three Sundays must not be consecutive but rather have 14 days between two odd-numbered Sundays. A month can have a maximum of 31 days. Let the first Sunday be $x$, then the second Sunday is $x+14$, and the third Sunday is $x+28$, so $x+28 \leqslant 31$,

Solving for $x \leqslant 3$;
Thus, the first Sunday can be the 1st or the 3rd.
If the first Sunday is the 1st, then the 20th of the month is a Friday;
If the first Sunday is the 3rd (in this case, the month has 31 days), then the 20th of the month is a Wednesday.
Therefore, the 20th of the month can be a Friday or a Wednesday (in this case, the month has 31 days).