Allah (SwT) has said,
Lord! Surely You are the One Who gathers men on a Day about which there is no doubt; surely Allah will not fail His promise. (Qur’an, 3:9)
Al-Jami’ is an Attribute of Allah (SwT) derived from jam’جمع, its root word, which means: to gather, assemble, horde, or combine. The Day of Gathering is the Day of Judgment. It is called so because Allah (SwT) will gather on it the early generations of His creation and the last ones, the jinns and mankind, all the residents of the heavens and the earth, each and every servant of Allah (SwT) and his deeds, every oppressor and those whom he oppressed, and every prophet and those to whom he was sent.
He will also combine between the rewards of those who obeyed Him and the penalty of those who disobeyed Him.
The subject of jam’, gathering or combining, is referred to in several verses of the Holy Qur’an such as the following:
Allah: there is no god but He; He will most certainly gather you together on the Resurrection Day; there is no doubt about it. (Qur’an, 4:87)
He has ordained mercy on Himself; most surely He will gather you on the Day of Resurrection, there is no doubt about it. (Qur’an, 6:12)
And on that Day, We will leave some of them in conflict with others, and the trumpet will be blown, so We will gather them all together. (Qur’an, 18:99)
Say: Our Lord will gather us together, then will He judge between us with the truth, and He is the greatest Judge, the all-Knowing. (Qur’an, 34:26)
Say: Allah gives you life, then He causes you to die, then He will gather you to the Day of Resurrection wherein there is no doubt... (Qur’an, 45:26)
And one of His signs is the creation of the heavens and the earth and the living beings whom He has spread forth in both, and when He pleases, He is all-Powerful to gather them together. (Qur’an, 42:29)
On the Day when He gathers you for the Day of Gathering, that is the Day of loss (for some) and gain (for others): Whoever believes in Allah and does good deeds, He will remove from him his evil and cause him to enter gardens beneath which rivers flow to abide therein forever; that is the great achievement. (Qur’an, 64:9)
One whose knowledge is perfected and whose conduct is good deserves to be called jami’. For this reason, a perfect person is one who does not put out the light of his piety by the light of his knowledge; so is one who combines perfect vision with foresight.