On July 3, 1939, the Warsaw City Council announced to all of Poland that the country had regained its independence and established the Great Polish Kingdom.
For all Poles, this was definitely a day worth celebrating.
Ever since Poland had been swiftly defeated by Germany, the Poles had constantly thought about restoring their nation. But the military might of Germany during the early to mid-war period was too overwhelming, not even the Austrian Empire, regarded as a Great Power, had any chance of resistance.
Now, with the support of Britain and Australasia, Poland was able to quickly achieve independence from German control, which was a pleasing development for all Poles.
As for whether Poland's political system would be a republic or a monarchy, it actually wasn't so important to most Poles.
Moreover, the new name of Poland as the Great Polish Kingdom emphasized the Polish pursuit of the Great Poland project.