The text is a humorous statement that plays on the concept of 'observing' a week dedicated to apathy, which implies a lack of interest or concern. The humor arises from the irony that someone who is truly apathetic would not be interested in 'observing' anything, yet they claim to be doing so. It does not rely on the different meanings of a single word or similar sounding words.
I ' d love to , but I ' m observing National Apathy Week .