Today the Federal Register published the two presidential actions of January 20, 2017 signed after the inauguration. One was Executive Order 13765, which I have already written about.
The other was Proclamation 9570 – Day of National Patriotic Devotion.
I have provided the direct link to the full text because it is the first proclamation of this presidency. Readers here can conclude for themselves what the meaning of and purpose for it might be, and I will refrain from commenting on it.
Based on the order of publication (page numbers in the Federal Register), the proclamation appears to be the very first signed action of this president.