It may be of interest to read a letter or two from distinguished persons along these lines. Here, for example, is the delightfully informal way in which Thomas Bailey Aldrich invited his friend William H. Rideing to dinner on one occasion:[1]
April 6, 1882.
Dear Rideing:
Will you come and take an informal bite with me to-morrow (Friday) at 6 p. m. at my hamlet, No. 131 Charles Street? Mrs. Aldrich and the twins are away from home, and the thing is to be sans ceremonie. Costume prescribed: Sack coat, paper collar, and celluloid sleeve buttons. We shall be quite alone, unless Henry James should drop in, as he promises to do if he gets out of an earlier engagement.
Suppose you drop in at my office to-morrow afternoon about 5 o'clock and I act as pilot to Charles Street.
Yours very truly,
T. B. Aldrich.
[[1]] From "Many Celebrities and a Few Others—A Bundle of Reminiscences," by William H. Rideing. Copyright, 1912, by Doubleday, Page & Co.
And one from James Russell Lowell to Henry W. Longfellow:[2]
Elmwood, May 3, 1876.