I don’t know what your argument was about, but if it was anything like most arguments between parents and their adult children, it ultimately had to do with what you mention: separation—that is, renegotiating your roles as your son gets older. Indeed, your son just got married, and though you adore your daughter-in-law, this does represent a new life phase for him, one in which his primary relationship is the one he has with his wife. That doesn’t mean you two will necessarily be less close—plenty of married adults are very close with their parents—it just means that you have the potential to develop a healthier and less fraught kind of closeness. In other words, this is a wonderful opportunity.
Most children, with or without siblings, go through a process way back in toddlerhood called “separation and individuation.” Separation refers to the child’s recognition that he or she is a separate person from the parent (as opposed to the infant’s concept of “fusion,” in which they’re part of the same whole). Individuation refers to the child having his or her own thoughts, feelings, and ways of seeing the world that may be different from the parent’s. If all goes well in this process, the child feels connected to the parent, but also free to be his or her own person.
Sometimes, though, parents and children become “enmeshed,” usually because the parent—due to her own history, or her not getting her needs met in other ways—struggles to let go. With separation and individuation, the parent and child feel both comfortably connected and separate. With enmeshment, however, it becomes hard to distinguish one person’s emotional experience from the other’s: The parent tends to be over-involved in the child’s life (for instance, trying to direct the son’s or daughter’s choices), making it hard for the child to become independent. The thing about enmeshment is that it can be confused with closeness. And when there’s a breach, it feels particularly painful.
All healthy relationships go through rupture and repair. There’s a rupture—a misunderstanding, an unfortunate interaction, hurt feelings—followed by a repair. The repair might involve one person or both people taking responsibility for their actions, making a genuine apology, or working through a difference. I have a feeling that your son was also upset by what happened on Mother’s Day but instead of taking into account his experience and what you can do to repair your part, you’re insisting that he re-create the Mother’s Day you wanted—as if a do-over consisting of him grudgingly restaging the day’s festivities would repair this for either of you.
If you can see beyond the hurt, you might view your son’s walking out not as “the meanest thing a child can do to a mother,” but as an indication that he wants to preserve your relationship. The meanest thing a child can do isn’t leave during a chaotic argument so that both of you can calm down and not do even more damage; the meanest thing isn’t being willing to move forward despite the extremely hurtful things that were said; the meanest thing isn’t his saying, after six weeks of hearing you attempt to get him to accede to your demand, that he feels those conversations need to end because they aren’t helping either of you. He’s essentially saying, I’m not giving up on us, Mom, but I care too much about our relationship to argue about this anymore—let’s agree to disagree, as all people must do.