Today and tomorrow are called פורים קטן, the “little” Purim. This occurs when there is a leap year with two Adars. Purim is celebrated in the second month of Adar.
Purim Katan is considered a happy day. As a result, we do not say תחנון and we are not permitted to declare a fast on these two days. It is also forbidden to give eulogies on these days.
At the time of the Sanhedrin, a leap year could still be declared up to the end of Adar. In a situation like this, where Purim was already celebrated, and a leap year was declared later, Purim would need to be observed a second time in the second Adar.
Meanwhile, today and tomorrow we can greet our friends with a פורים קטן שמח!