I tend to try not to bang on about Mother's and Father's days, as it's a bit odd for me. My parents divorced when I was about 2, I had a great father figure in my young life, I gained step-parents in my teens. I've always found it a bit tacky saying "Thanks" or "I love you" on a set day of the year. So, that's 5 parental figures, and my own stubborn reaction against guilt-trip attempts. Also, I'm now orphaned, but with amazing step-parents. Unpick that mess!
However, I recently saw someone suggest using Mother's Day to thank a carer you may not even realise you have.
Not-mums can arise for many reasons. It may be that you no longer have a mum. Or they don't support you in ways you need. Or you do love them but live miles apart and someone geographically closer is a rock for you. Perhaps you're part of a very close knit family with an amazing aunt.
I like the idea of thanking Not-mums, as they often care deeply without obligation or expectation, and from choice rather than through blood. Is Mother's Day the right day to do it? I think it's a great idea to recognise and share some love, and perhaps the sentiments around Mother's Day do make it a good day to say "Mother's Day made me think of you, even though you're not my mum."