Day 25 ~ Memory

Why have I always remembered numbers, and particularly dates, so well? Today I saw "October 15" on the calendar and thought: "Today is Teresa's saint's day. And tomorrow is her birthday." We were around my son's age (14) when I interacted with her. For a year or two we spent many weekend afternoons together, but that was a long time ago. Like Teresa's, I remember many other birthdays from people who mentioned their special dates in passing. Conversely, I often forget names. What is the essential difference between letters and numbers?