I'd like to share a technique I've been using for the past couple of years that has been super fun and useful.
I have built and assigned a memory palace/loci route for every month of the year (so 12 in total) such that every peg in those routes corresponds to exactly one day in the year.
To make it more seamless to remember which month corresponds to which palace I assigned them somewhat logically, so e.g. the december palace corresponds to a place where I usually stay during christmas, while summer months may correspond to places where I have made a summer vacation or places that are quite hot, etc.
Now, with this technique I don't use a calendar anymore. Every morning I wake up I visualize the peg of the corresponding day in the memory palace of the corresponding month and retrieve in my mind any birthdays and important appointments. Conversely, when I get to know a new birthday date of a friend or need/want to remember any other date or appointment, I visualize my calendar palaces and store it at the correct peg (linking it both to the locus, as well as other information already stored there).
For less important appointments and short-term todos that I don't need to keep for long, I use two other concepts in parallel: several "todo" palaces that I constantly overwrite as items come and go, and a simple week palace with one peg per day of the current week.
Has anyone done the same, and if so, what are your experiences?