Djeggnog,
While your calendar that you printed here is somewhat correct, Passover is observed on Nisan 15, not on the 14th. It is an 8 day festival, beginning with a Seder that begins at sundown on the 14/15th and merges with the 7-day Festival of Unleavened Bread on the 15th.
First, the Jehovah's Witnesses are following a lunar calendar that they made up, not the same used by the Jews which uses the astronomical New Moon (and not a visible one) to determine the beginning of a month. There is no Sanhedrin in Jerusalem to determine when a visible New Moon begins (which was an unreliable, impractical, and if it occurs during a cloudy night, imprecise manner to determine a month's start). The Governing Body of Jehovah's Witnesses has taken it upon themselves to act as the Jerusalem Sanhedrin and pretend they are watching Jerusalem's sky (which they are not). You cannot determine in advance when a New and Full Moon should be discerned from Jerusalem if it has not happened yet.
Next, and most important, we are talking about the merging of two feasts. Literalist Christians, like the JWs, refuse to acknowledge both Jewish tradition and secular history that shows that Passover predated the Exodus. It merged with the 7 day festival in observance of the Exodus from Egypt to create the holiday you see today. (See Hebcal.com at https://www.hebcal.com/home/1247/pesach-on-15th-of-nissan-vs-the-14th for more info).
As I write this it is Nisan 14, the day of preparation. After sundown we (already) mark(ed) the day by searching for the last of the chometz (leaven) by candlelight. We then observe the Passover with a Seder the following evening on the 15th of Nisan.
This year the 10th of March after sundown marks Nisan 15. Tuesday, when the Jehovah's Witnesses hold the Memorial will be Nisan 16.
Jehovah's Witnesses know very little about what they are doing. Remember, this religion was started by white, Gentile men in America at a time when anti-Semitism was an earmark of Christianity. Jewish "notions" were once rejected by all Christians until the Holocaust, so JWs felt it was their Christian right and duty to "restore" the Jewish calendar as they saw fit.
After the Holocaust, Christianity in general recognized that its own shortsightedness and bigotry contributed indirectly and sometimes directly to the Nazi's genocide of the Jews. Thus afterwards Christian scholars accepted Jewish academia as authentic and authoritative.
This, however, never changed groups like the Witnesses. To this day they even claim themselves as the main target of the Holocaust. So while they observe their Memorial, claiming it is Nisan 14 like a bunch of April-fool idiots, it will actually be the second night of the Passover holiday, Nisan 16, on the Jewish calendar. We call the entire feast Pesach or Passover now.
To determine the actual date on the Jewish calendar anywhere, visit Hebcal.com regularly.