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 day which is <WSD>not</WSD> a Saturday , Sunday , or legal holiday . 
Meaning of the word marked with <WSD> tags:  negation of a word or group of words