The first period lasted for forty years, from the time he was born in 5640 (1950) until his father the Rashab rose to a higher level of holiness and good in 5680 (1920). His father, the Rashab, was the leader of the generation, similar to The Era of the Forefathers - Avraham, Yitzchak, and Yaakov. The Rebbe Rashab established the "Tomchei Tmimim Yeshiva" and went full speed ahead with the dissemination of the wellsprings (teaching the deepest secrets of the Torah) to everyone, even to those who are outside and not part of the Jewish community circles. This was done as a preparation for the time when his son the Rayatz would eventually become the Lubavitcher Rebbe after him, similar to our forefathers who prepared the world before the giving of the Torah.
The second time period starts from the time when the Rayatz became the Rebbe and continued his job for the next thirty years. The Rayatz reached and taught Judaism and Chassidut once again to even more people (and to the furthest places) than his father did. He taught Chassidut to people who didn't even know Hebrew and translated it into their own language. (G-d also gave us the inner dimensions of the Torah in addition to the Torah itself). This was especially true during the last ten years of his life in America, a place up until now didn't experience "The giving of the Torah". The Rayatz came and made "The giving of the Torah" happen there also. He taught the revealed parts of the Torah as well as its inner dimensions.
The third time period started after the Rayatz rose to a higher level of holiness and good. The Lubavither Rebbe becomes the new Rebbe. The Rebbe describes this new era:
"This third period, a continuation of the Rayatz's work after he rose to a higher level of holiness and good, is a time when the wellsprings of Chassidut are reaching more and more people around the world, until the job is done. Everything is already in place and ready for the feast that will take place in the near future - Yemos Hamoshiach.
Since the Rebbe Melech Hamoshiach began his job, thousands of "shluchim" (Chabad Rabbis of Chabad Houses) have taught Chassidut all around the world, and thank G-d have been very successful at their jobs and have completed the jobs the Rebbe gave them. Today, we are standing at attention and are ready for the redemption. Therefore, the Rebbe refers to this time period as "Yemos Hamoshiach"!"
