I've read an article about the above few months back. It's a very creative concept of "sentimental" memory database. Most of us probably do have our own diary or blog to jot down the important occasions in our day to day living, things like birthday, the date we bought our new car or house, the day our son/daughter being accepted to the university and others.
The concept of memory jar is basically the same. The only difference is that all the "sweet" moments will be written down on a piece of paper and to be kept inside the jar. The writer do suggest that the jar to be opened during special gathering for example family reunion or birthday .
I'm planning to have one very soon.