It is not a mistake in counting the day from sundown to sundown. Here is the explanation from the horse's mouth.
*** w90 2/15 15 From Seder to Salvation ***
25 The Governing Body of Jehovah’s Witnesses establishes the date for the Lord’s Evening Meal in line with the ancient method. Nisan 1 is determined by when the new moon nearest the spring equinox can likely be observed
at sunset in Jerusalem. Counting 14 days from that brings one to Nisan 14, which usually corresponds to the day of the full moon. (See The Watchtower
of June 15, 1977, pages 383-4.) On the basis of this Biblical method, Jehovah’s Witnesses around the globe have been advised that the celebration of the Memorial this year will be after sunset on April 10.
*** w77 6/15 383-4 Questions from Readers ***I understood that the Memorial celebration was always on the evening of a full moon. But in 1977 the Memorial was on April 3, and my calendar gave April 4 for the full moon. Why the difference?
Often the celebration of the Lord’s Evening Meal and the full moon do coincide, but not always. There may, for example, be a day’s difference, depending on where you live and the calendar used.
To see why this is so you need to understand the basic method presently used by the Governing Body of Jehovah’s Witnesses in establishing the date for the annual Memorial celebration.
It was on the 14th of the Jewish month Nisan, the date of the Passover, that Jesus directed his followers to commemorate his death. (Luke 22:14-20) Appropriately, the date for the Memorial celebration is arrived at as the Jews back then determined the date for the Passover. They began the month of Nisan when they could first see the new moon in the spring nearest the equinox. Passover came fourteen days later.—Isa. 66:23; Ex. 12:2, 6.
Jehovah’s Witnesses now follow this ancient pattern in determining the Memorial date. Please note that the first thing that needs to be established is when the new moon nearest the spring equinox (about March 21) will be visible in Jerusalem. This is not the astronomical new-moon time listed on a calendar or astronomical table. Why? Because the first thin sliver of the new moon is not visible until eighteen to thirty hours after the moment of astronomical new moon.
Let us take 1977 as an example. In determining the matter months beforehand so as to inform the congregations earth wide, the Governing Body of Jehovah’s Witnesses had to calculate when the new moon would become visible in Jerusalem. From the astronomical standpoint the instant of new moon was at 20:33 (8:33 p.m.) Jerusalem clock time on March 19, 1977. Of course, that new moon was not then visible nor would it be for many hours. But could the first sliver of the new moon be seen the next evening about sunset? Because of various involved factors it seemed unlikely that the moon could be sighted in the rather bright sky west of Jerusalem about sunset on March 20, 1977. Consequently, the Governing Body settled on March 21, 1977, as the time when, with assurance, the new moon would be in position to be seen from Jerusalem about sunset. Nisan 14 would thus commence at sunset on April 3. That is when Jehovah’s Witnesses held the Lord’s Evening Meal. What about the full moon that month? When was it?
Astronomical tables list the full moon as occurring on April 4 at 04:09 (4:09 a.m.), Greenwich (England) Mean Time. But at that same moment if you live in another time zone obviously your clock would show a different time. For instance, Stockholm (Sweden) and Rome (Italy) are in the next time zone to the east of Greenwich. Hence, for them the moment of full moon occurred on April 4 at 05:09 (5:09 a.m.). New York (U.S.A.) and Lima (Peru), though, are five time zones to the west of Greenwich, so for them the full moon was at 23:09 (11:09 p.m.) on April 3, 1977. Because of this variation according to time zones some 1977 local calendars indicated that the full moon occurred on April 4 and others said April 3.
In any case, the basic point to appreciate is that the date for the celebration of the Lord’s Evening Meal is determined by the new moon (visible in Jerusalem), not the full moon. Nonetheless, the Memorial falls fourteen days after the appearance of the new moon. Thus it always occurs about the time of the full moon. It is good to know this in case some of Jehovah’s Witnesses are cut off from contact with the Governing Body and do not know what date for Memorial has been determined. In that situation, if they observed the Memorial on the evening of the calendar date for the full moon after the spring equinox, they would likely be celebrating it on the same date as the rest of their brothers or at least very close to it.