How can rabbis today make a new holiday with special holiday prayers like Hallel on Israel’s Independence Day?

This question has been addressed at length by many halachic authorities, such as Rabbi Shlomo Goren, Rabbi Yehuda Gershoni, Rabbi Moshe Tzvi Neriah, Rabbi Meshulem Roth, and others. Their responses are compiled in the book, "Laws of Independence Day and Jerusalem Day," by Professor Nachum Rackover. Rabbi Meshulem Roth wrote: "Behold, there is no doubt that this day which was established by the State and the members of the Knesset, who were chosen by the majority of the congregation, and by the majority of the great rabbis, to celebrate it in the entire Land in memory of the miracle of our salvation and freedom – it is a mitzvah to commemorate it by making it a joyous day with the recital of Hallel." Rabbi Menachem Kasher stated: "That everyone who says Hallel on this day in accordance with the great rabbis is certainly doing a great thing." As we say at the end of the Yom Ha'azmaut prayers, "Just as we merited the beginning of the redemption, so may we merit the completion of this redemptive process," speedily, in our day .