The Feast of Revelation (Yearly Sabbath) is one of the Holy Days that God commanded to be kept forever in Leviticus chapter 23 in the Bible.  Its significance and how to calculate its yearly date can be found in THOTH, Essays 1, Tablet 14, "Feasts and Holy Days of the Lord" (Note: The point before the Feast of Revelation, the end of the Feast of Unleavened Bread, is also quoted below in order to understand the time frame of reference):

6 - On the seventh day (twenty-first of the first month) is a holy convocation with no servile work.

7 - After this holy convocation, the next day will be the offering of the "firstfruits" (as explained in Leviticus 23:10, which no longer is necessary; Christ was the firstfruit!).  After this day, count fifty days (seven weeks, which will include seven Sabbaths); another yearly Sabbath, a holy convocation, with no servile work.  Each week of these seven weeks symbolizes one seal of the seven seals in the Eternal Divine Path.  After the seventh seal, the last Revelation of God will (has) come.  Therefore, this yearly Sabbath from now on will be called the Feast of Revelation. 
 

This year's Feast of Revelation will be on Shakti 13, or June 1 (6 PM May 31 to 6 PM June 1).  Keep it as you would the weekly Sabbath, with no servile work (if possible), fasting, rest, spiritual practice, focus and contemplation on God, etc.  It is also a good time to review and meditate on the Eternal Divine Path and its seven Revelations and Truths that were revealed over the last 12,000 years of history, especially the seventh Revelation brought by Maitreya which is now on earth.

The reason it is called the Feast of Revelation is that: In the Jewish tradition, it is believed that the Torah was revealed 50 days after the Passover.  So this Yearly Sabbath was the Feast of Revelation.  However, with the Revelation of the Eternal Divine Path, we now understand what God meant by counting "seven" Sabbaths and so seven weeks.  Each was meant to symbolize one Seal, and the Feast of Revelation symbolizes the opening of the Seventh Seal.  Now it can be known as the day of Revelation of the last and the most complete Revelation of God!

Always remember that the spirit and ideation behind celebrating God's Holy Days is much more important than the specific rituals, dates, etc.  We must not become rigid and dogmatic as other religions have.  However, best is to have both the spirit behind the Day, and the correct date!