It is indeed not only a ridiculously dumb idea but makes absolutely no sense in universe to recall every ship in the fleet which could be hundreds of lightyears away doing vital work such as evacuating colonies, dealing with sensitive border disputes, delivering urgent medical supplies to meet for this arbitrary event.Frontier day happens every year but this is a big anniversary apparently. The entire Starfleet is assembling in the Sol system, that's never happened before, thus unprecedented. It's also an incredibly stupid and dangerous idea in universe. We see Geordi complaining about how the higher up brass are ignoring his warnings. It's almost certain the highest ranking admirals have been replaced by changelings and are setting up the celebration for the attack.