If having children is in the plan, I will comment that having kids has been much more Schellingy than getting married was, and any life-changes I didn't have solidly cemented before having kids damned sure weren't going to be made after having them.
One thing I'm glad I did was make a habit of trying to write fiction a few times a week before the kids came along - basically, selfishly carve out some me-time - because now I'm still able to maintain this habit with two babies.
Children might be a common time start resolutions, especially ones that might effect the children, but I disagree that it is more Schellingly in the sense that if two people were to choose a day in my, and try to name the same day, the wedding would probably be third only to birth and death.
But I guess that does not make it the best day to start a resolution.
However, note that in my position, it is better for me to temporarily believe that this is the best time to start a resolution, because that makes me more likely to go through it. Thanks for the basilisk. :)
Also, I already kind of used up the first kid, by setting that as my deadline for deciding about cryonics.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.