MEMORIAL CELEBRATION

Jehovah's Witnesses commemorate Christ's death as a ransom or "propitiatory sacrifice" by observing the Lord's Evening Meal, or Memorial. They celebrate it once per year, noting that it was instituted on the Passover, an annual festival. This is the only celebration the Bible commands Christians to observe..
Of those who attend the Memorial, a small minority worldwide partake of the unleavened bread and wine. This is because Jehovah's Witnesses believe that the majority of the faithful have an earthly hope. Only those who believe they have a heavenly hope, the "remnant" (those still living) of the 144,000 "anointed", partake of the bread and wine. In 2014, the number of persons who partook worldwide was 14,121, whereas the number who attended was 19,950,019.
The Memorial, held after sunset, includes a talk on the meaning of the celebration and the circulation among the audience of unadulterated red wine and unleavened bread. Jehovah's Witnesses believe the bread symbolizes Jesus Christ's body which he gave on behalf of mankind, and that the wine symbolizes his blood which redeems from sin. They do not believe in transubstantiation or consubstantiation. Because many congregations have no members who claim to be anointed, it is common for no one to partake of the bread and wine.
See also: What is the meaning of the bread and the wine served at the Memorial?
Jehovah’s Witnesses observe the Memorial after sundown on Nisan 14, according to the reckoning of the Jewish calendar that was common in the first century. The Jewish day begins at sundown and extends until the following sundown. So Jesus died on the same Jewish calendar day that he instituted the Memorial. The beginning of the month of Nisan was the sunset after the new moon nearest the spring equinox became visible in Jerusalem. The Memorial date is 14 days thereafter. (Thus the date for the Memorial may not coincide with that of the Passover kept by modern-day Jews. Why not? The start of their calendar months is set to coincide with the astronomical new moon, not the visible new moon over Jerusalem, which may come 18 to 30 hours later. Also, most Jews today keep the Passover on Nisan 15, not on the 14th as did Jesus in harmony with what was stated in the Mosaic Law.)
Memorial is always held on the first full moon after March 21st. It's always a full moon because Memorial is always held 14 days after the new moon. Hebrew months start on the new moon (the new month). Therefore Nisan 14 is always a full moon (+- 24 hours).
See also Why Is Memorial of Christ Observe Annually?