From hospital stays to Shabbat dinners, a few lines a day became a Jewish way of reckoning with memory, loss — and the comfort of routine. In January 2011, I wrote my first entry in a five-year diary.

This is One Thing, a column with tips on how to live. I’ve made the promise to myself so many times: I will keep a diary. I like the idea that one day, decades into the future, I can turn over the ...