I don't keep a calendar but I keep track of it in my head. Although I do it MUCH less than I used to. I think I will be officially 'healed' of this SSM stuff when I no longer do that!
I used to make a mental note, such as "This is the first week of February" and then the next time we ML I would think, "This is the last week of April". I never actually marked it down on the calendar but I kept track that way. I still keep track of it mentally but thank GOODNESS it is on a weekly basis now. I started doing that when H agreed to twice weekly and then was not realizing that we weren't meeting that. It was at that point that I had to inform him that if we were to ML twice a week then not more than 3 days can go by without us meeting our goal. (all you math people out there will be able to shoot holes in that statement but fortunately H couldn't) Basically what I think he wanted to do was to take the week off and only ML on the weekends which was really not what I had in mind.
I wanted to incorporate ML as part of the fabric of our lives, not solely as a quota or something to fit in whenever time permitted. So for us, incorporating the 3 day rule has worked pretty good. Note that there are TONS of weeks where we go longer than 3 days, but it was necessary in the beginning when we were getting things back on track. Now I know (and he knows) that if we have a week where we don't ML twice that the next week will be fine. There was a time when I didn't trust him as far as I could throw him and so I held him strictly to the 3 day rule. (sounds romantic huh!) I don't know how he put up with me half the time.
So I guess it is a part of being HD...remembering with remarkable accuracy the last time you ML. Just like I am sure that my H could tell you the exact date that he last changed the oil in the car.