I suppose that if there were ever to be a National Patent Prosecution Day, it would have to be celebrated on May 2nd. After all, that is the anniversary of the Topliff v. Topliff decision in which the Supreme Court of the United States acknowledged:
The specification and claims of a patent, particularly if the invention be at all complicated, constitute one of the most difficult legal instruments to draw with accuracy, and in view of the fact that valuable inventions are often placed in the hands of inexperienced persons to prepare such specifications and claims, it is no matter of surprise that the latter frequently fail to describe with requisite certainty the exact invention of the patentee, and err either in claiming that which the patentee had not in fact invented, or in omitting some element which was a valuable or essential part of his actual invention.
Topliff v. Topliff, 145 U.S. 156, 171 (1892).
I suppose another candidate would be June 16th. That is the anniversary of Diamond v. Chakrabarty:
The Committee Reports accompanying the 1952 Act inform us that Congress intended statutory subject matter to “include anything under the sun that is made by man.” S. Rep. No. 1979, 82d Cong., 2d Sess., 5 (1952); H. R. Rep. No. 1923, 82d Cong., 2d Sess., 6 (1952).
Diamond v. Chakrabarty, 447 U.S. 303, 100 S. Ct. 2204, 65 L. Ed. 2d 144 (1980).