– “M” might be a son or daughter (“Mike Jr.”) or even a dog (“Max”). The announcement could be: We are now a family of two — me and M. That would be sad but also tender — the smallest possible unit of love continuing on.
Given the mix of what looks like a possible username, the word “widow,” “sad announcement,” and “m new,” this might relate to an online post from someone recently widowed who is trying to move forward (“m new” could mean “I’m new” or a new relationship/status).