The month of February of a given year has five Fridays. What day of the week is January $$31$$ of that year? 
Overseas Competition->Knowledge Point->Word Problem Modules->Periodic Problems->Periodic Problems of Time
There are seven days in a week; so, in order to have five Fridays, a month has at least $$4\times7+1=29$$ days. Since February has at most $$29$$ days, the $$1^{\rm st}$$ and $$29^{\rm th}$$ days of this February must both be Fridays, which means that January $$31$$ is a Thursday. 