Just because they don't deliver mail on Sundays (at least not in the United States) doesn't mean that you can't send any. I actually consider Sundays to be my most productive writing days... probably because I work at a library on Sundays and have the kind of time, space and serene environment that are conducive to good letter writing. What about you, where and when do you do create your best correspondence?
Note: I received this wonderful postcard from PostMuse. It's titled "Good Post is Not a Thing of the Past."