Sentence:  If the last day ( due date ) for performing any act for tax purposes , such as filing a return or making a tax payment , etc. , falls on Saturday , Sunday , or a legal holiday , you may perform that act on the next succeeding <WSD>day</WSD> which is not a Saturday , Sunday , or legal holiday . 
Meaning of the word marked with <WSD> tags:  time for Earth to make a complete rotation on its axis