This will help to keep up with which crops were grown where. Doing this will help with crop rotation and determine which plants are grown in that spot next year.
If possible, keep tabs on when you planted, watered, fed, pruned, or harvested the plants in your garden. Then, when looking back you can clearly see how often you performed certain tasks and how often.
In our gardens we have control over some things, however we don’t have control over the weather. One year we may have a scorcher of a summer and a mild winter, and the next we might have a sweltering spring and bitterly cold and long winter. So, I recommend including a simple summary of the weather too, just to know the conditions that your plants were growing in.