Wikipedia writes:
Quds Day (Jerusalem Day, Quds is the city's Arabic name), officially called International Quds Day (ﺭﻭﺯ ﺟﻬﺎﻧﯽ ﻗﺪﺱ) in Iran, is an annual event held on the last Friday of Ramadan that was initiated by the Islamic Republic of Iran in 1979 to express solidarity with the Palestinian people and oppose Zionism as well as Israel's control of Jerusalem. In Iran, the government sponsors and organizes the day's rallies. Quds Day is also held in several countries in the Arab and Muslim world with protests against Israel.

Quoting the Invitation of Imam khomeini, wikipedia continues:


"I invite Muslims all over the globe to consecrate the last Friday of the holy month of Ramadan as Al-Quds Day and to proclaim the international solidarity of Muslims in support of the legitimate rights of the Muslim people of Palestine. For many years, I have been notifying the Muslims of the danger posed by the usurper Israel which today has intensified its savage attacks against the Palestinian brothers and sisters, and which, in the south of Lebanon in particular, is continually bombing Palestinian homes in the hope of crushing the Palestinian struggle.

I ask all the Muslims of the world and the
Muslim governments to join together to sever the hand of this usurper and its supporters. I call on all the Muslims of the world to select as Al-Quds Day the last Friday in the holy month of Ramadan — which is itself a determining period and can also be the determiner of the Palestinian people’s fate — and through a ceremony demonstrating the solidarity of Muslims world-wide, announce their support for the legitimate rights of the Muslim people. I ask God Almighty for the victory of the Muslims over the infidels.
~Ayatollah Khomeini

Imam Khomeini’s Quotes: Palestine and Al-quds
• All must know that the superpowers aim in creating Israel does not end in the occupation of Palestine.

They plan, Heaven forbid, extending the fate of Palestine to all Arab countries.

• Do not the authorities of the nation see or know that diplomatic talks with history's criminal and powerful politicians cannot save Lebanon, Palestine or quds and the number of crimes and oppressions increases daily.

• It is the intonation of Allah-o-Akbar, the cry of our nation that disappointed the Shah in Iran and the usurpers in Palestine. Essentially, it is the duty of all Muslims to liberate the quds and remove this nucleus of evil from Islamic lands.

• Jerusalem is the first Qiblah of Muslims and it belongs to them.

• Muslims of the world should regard the quds Day as All-Muslims Day or the oppressed People's Day.

• Neither the Iranian Muslim nation, nor any other Muslims and, as a matter of fact, no open-minded individual, will recognize Israel. We shall always support our Arab and Palestinian brothers.

• Quds belongs to the Muslims and must be returned to them.

• The issue of quds is not a private or personal issue. It is neither the exclusive problem of one country nor a present-day problem of all Muslims. Rather, it is a phenomenon concerning the monotheists and faithful people of all ages-past, present and future.

• The quds Day is a day in which Islam is to be revived.

• The quds Day is a day in which the destinies of the oppressed nations must to determine.

• The quds Day is a universal day. It is not an exclusive day for quds itself. It is a day for the oppressed to rise and stand up against the arrogant.

• The quds Day is Islama's Live Day!

• The quds Day is the Day of Islam.

• The quds Day which fall on the last days of the God's Great Month (Ramadhan) is a proper day on which all Muslims be freed from the bondage of the Big Satan (Great Devil, USA) and other superpowers
and join the Infinite Might of God.

• The quds Day which is coincidental with the Night of Destiny (Laylatul-Qadr) must be revived among the Muslims and be the starting point of their awakening and awareness.

• We are on the side of the oppressed which-ever pole they may be in. Palestinians are oppressed by the Israelites, therefore we side with them.

• We fully support our Palestinian brothers and those of southern Lebanon in their fights against the usurper Israel.

• We must all rise, destroy Israel and replace it with the heroic Palestinian nation.

* Source: rkhomeini.org