"What do we celebrate on the 4th of November?" is still a common search on Yandex. Day of People's Unity was first celebrated on November, 4th in 2005. What does "people's unity" actually stand for? Let's sort it out.
All of you have seen this monument on Red Square - two men, one is sitting, another one is stretching his arm (originally he stretched it towards the Kremlin urging to free it from invaders. But now the monument stands on another place and the meaning of his gesture is lost). These are Minin and Pozharsky - two leaders of the people's resistance to the Polish occupation of Moscow in 1610-1612. By the way, this is the first sculptural monument put up in Moscow.
So, after Rurikovichi dynasty had died out after many centuries of ruling, Tzars kept changing every several years while none of them was able to firmly establish himself as a monarch. At some point boyars (nobles) decided to make a radical step - they invited Polish prince to the Russian throne. Later they changed their mind, but the Poles were already here and in 1610 they took control over Moscow. There were no monarch and no government in the country, but people managed to stop feuding and to unite for the resistance to foreign invasion. Kozma Minin (a sitting man) organized donation collecting for militia armament, and Dmitry Pozharsky lead the storm of Moscow. Poles were besieged in the Kremlin (felt strange because this stronghold was designed for something completely opposite - for Russians to hide there from enemies), and on the 3rd of November all the invaders surrendered. The next day Moscow was returned to Russians.
Thirty five years later Tzar Mikhail Romanov proclaimed the 4th of November a holiday.
The Day of Unity was reestablished in 2005, in many ways with a purpose to replace another November holiday - Day of Revolution celebrated on the 7th of November, one of the greatest national holidays in Soviet time. It remained a day off and continued to be celebrated by communists after 1991 as well. But in 1996 Boris Eltzin renamed it to the Day of Accord and Reconciliation - between communists and their counterparts. And at the end of 2004 a new decree canceled Day of Reconciliation and introduced Day of Unity. The same decree prolonged New Year holidays from 2 to 5 days.
According to polls only a third of Russians can give a detailed answer to the question "what do we celebrated on the the 4th?" Luckily, there is Yandex.