In the UK, this Sunday is mother's day! Though I have made a point of celebrating as many holidays as possible since leaving, this will not be one to add to my list as my mother would be genuinely offended if I sent her a card or something.
Ask a JW why they don't celebrate holidays and they'll tell you it's because of some nefarious pagan connection. But this is really just a cover. The real reason is that not celebrating holidays is one of the ways in which JWs are set apart and made to feel special. They don't involve themselves in things Jehovah hates like all you pagans! But note, it builds this self worth in a way that's very different to how holidays do the same.
Celebrating mother's day gives Mum self worth based on her work as a mother, refusing to celebrate it gives her self worth by virtue of her membership in the group. Thus a JWs identity is, as we know, wrapped up very strongly with their group identity.
This is important, the reason JWs can be so cruel is because anything that threatens their place in the group directly threatens their own identity/ego, and so they push back against it.
Opposition to holidays are one of the key ways JWs build this identity around the group, and that is the real reason why JWs don't celebrate Mother's day. (or any other holiday)