If $$15^{}\text{th}$$April is Friday, what day of the week is $$1$$\textsuperscript{st~}April ? 
Overseas Competition->Knowledge Point->Word Problem Modules->Periodic Problems->Periodic Problems of Time
There are 14 days between $$1$$\textsuperscript{st~}April and $$15^{}\text{th}$$April.  14 days = 2 week  Therefore, $$1$$\textsuperscript{st~}April is Friday. 