In the Six Day War, the Jewish nation, with God’s help, achieved a tremendous victory over its enemies. The war began on three fronts, and in the span of just six days, we utterly shattered our enemies’ military strength and dealt them a total defeat. At the same time, we liberated all of the holy places in Judea and Samaria – most significantly Jerusalem and the Temple Mount – along with the Sinai Peninsula and the Golan. Anyone who viewed these events honestly, and had even the slightest spark of faith in his heart, saw clearly the words of our holy Torah, “For the Lord your God walks in the midst of your camp to save you and to deliver your enemies before you”(Devarim 23:15). This great victory was truly a manifest miracle.
In order to thank Hashem and publicize the miracle, the Chief Rabbinate, headed by Rabbi Isser Yehuda Unterman and Rabbi Yitzchak Nissim, established the twenty-eighth day of Iyar, the day on which Jerusalem and the Temple Mount were liberated, as a day of thanksgiving and joy for all of Israel. They also instituted the recitation of Hallel with a blessing after the morning services (Shacharit). Rabbi Shlomo Yosef Zevin and Rabbi Shaul Yisraeli also participated in and supported this decision11.
 See Rabbi Shmuel Katz’s article on the topic in HaRabbanut HaRashit, vol. 2, especially pp. 974-75. For the exact wording of the Chief Rabbinate’s decision, see Rabbi Rakover’s Hilchot Yom HaAtzmaut VeYom Yerushaliyim, p. 387. On page 125, Rabbi Rakover cites a teshuvah (responsa) from Rabbi Unterman, discussing the great importance of the mitzvah to publicize a miracle, which is possibly even greater than reciting the Shema; we even interrupt Torah study on Purim to participate in the reading of the Megillah, in order to publicize the miracle. There, Rabbi Unterman also expands upon the mitzvah of establishing a holiday on a day the Jews were delivered from distress. Rabbi Kaplan writes likewise (ibid. p. 204). See there, page 61, for an essay by Rabbi Diblitsky on the need to establish a day of thanksgiving for the Six Day War. According to Rabbi Ovadyah Yosef, however, one should not recite a blessing on the Hallel, for he holds, based on the Chida’s opinion, that we cannot institute the saying of Hallel with a blessing unless a miracle happens to all of Israel, and Rabbi Ovadyah does not view the Jews of Zion as the entirety of Israel.