For tasks with an "every few days" nature, it would be handy to have a calendar with, say, a box for each three days.
But you can adapt this method to a regular calendar by marking X's on that day and connected days when you do the action.
So for something like resistance training that you want to do every four days, you can "catch up" on the second, third or fourth unmarked day, but if you go more than four days, the chain is broken.
Note that you can potentially go a week between sessions by being early once then late the next time, so it's a little different than a calendar with multi-day intervals. But that extra flexibility might be helpful for some tasks.
But you can adapt this method to a regular calendar by marking X's on that day and connected days when you do the action.
So for something like resistance training that you want to do every four days, you can "catch up" on the second, third or fourth unmarked day, but if you go more than four days, the chain is broken.
Note that you can potentially go a week between sessions by being early once then late the next time, so it's a little different than a calendar with multi-day intervals. But that extra flexibility might be helpful for some tasks.