CuriousAboutJW, the Witnesses 'observe' the Memorial on the Jewish holiday of Passover (or of the the Passover Seeder?) on the Nisan 14th date of the Jewish religious lunar calendar, and as a result it does not fall on the same weekday each year. The early Jewish Christians (and some other Christians for at least a few centuries) also observed it on that day of the Jewish religious calendar. Other Christians chose to distance themselves from Judaism by a greater extent by observing it on what is called Good Friday.
According to the gospels Jesus instituted the observance on the Jewish Passover day of Nisan 14th (except that some scholars say that according to at least one gospel account the date was on Nisan 15th). In the USA the date of the WT's Memorial event typically is on the same day that USA calendars say the Jewish holiday of Passover is on.
Since the majority of JW don't partake of the emblems, the WT uses the word "observe" in regards to the event in the sense of "see" instead of the in the sense of "do". Most JWs don't do what Jesus said to do in regards to the event, namely to partake of the emblems (to drink the "fruit of the vine" and eat the unleavened bread). Instead they merely see the emblems (and hear a talk about them) and pass them onto another JW.