The Jewish religion views participation in non-Jewish religious practices very seriously. In fact, the first three of the Ten Commandments all essentially revolve around the same issue - idolatry is forbidden.
Furthermore, the Mishnah and the Talmud devote a full tractate to idolatry - Avodah Zara - going over the intricate details of what constitutes idolatry and what doesn’t and how Jews should conduct themselves in their relations with gentiles: i.e., in such a way that not only are they not worshiping other gods, but do not seem to be worshiping or assist others - even gentiles - in worshiping idols. According to the Mishnah the punishment given for idol worshiping is a sentence of death by stoning (Sanhedrin 7:4).
During the Roman rule of Palestine in the Second Temple period, the Sanhedrin stopped sentencing persons to death, apparently as this was only the prerogative of the Roman state. Since then, the sentencing of Jews to death for religious infractions is not practiced and the punishment is believed to be carried out by divine retribution.
So idol-worshiping is a very serious matter and if taking part in Valentine's Day is a form of idol worshiping, then it is definitely forbidden by Jewish law. But how do we determine this?
Unfortunately the Bible, Mishnah and the Talmud are all mute on the subject of Valentine’s Day, as this holiday is a much, much later contrivance; even a Roman precursor to it, Lupercalia – an ancient fertility cult celebrated February 13-15 – is not mentioned in these texts. Other Roman holidays are mentioned by name, however, with participation banned even in the loosest sense.
Before one can determine whether the Jewish faith sees sending valentines and giving chocolate on February 14 as a deadly sin, we must first see what Valentine's Day is and from whence it comes.
There are several early martyrs called St. Valentine. The one venerated on February 14 is Valentine of Rome, who was martyred in 496 C.E. Hardly anything is known about him other than the fact that he was a priest and that he was buried at the Via Flaminia. His saint day was documented very early on, but was taken out of the official Catholic calendar in 1969, due to a lack of information about his life.
For most of this time, the veneration of St. Valentine had nothing to do with romantic love – that was a later addition, begun in 14th century England with a line from English poet Geoffrey Chaucer's Parlement of Foules (1382): “For this was on seynt Volantynys day, Whan euery bryd comyth there to chese his make” (For it was on Valentine's Day, when every bird comes there to choose its mate).
It is highly unlikely that Chaucer’s birds were mating on February 14, however, as this happens during the spring and may in fact be another St. Valentine altogether.
This line from Chaucer's text and this idea of birds mating on Valentine's Day began spreading amongst European poets at this time, coinciding with the rise of the notion of courtly love. Over time and through these poems, the idea spread that February 14 - Valentine's Day - was a day devoted to romantic love, and lovers began sending letters of affections on this day. The oldest surviving Valentine letter is a poem written by Charles, Duke of Orleans in the 15th century.
The tradition of sharing Valentine's Day cards became exceedingly popular during the 19th century when the price of sending letters dropped and printers began producing cards for mass consumption. During the latter half of the 19th century and to a greater extent in the 20th century, the celebration of Valentine's Day spread outside of the English-speaking world and is today widely celebrated as a day of romance in practically every country around the globe, including Israel.
With these facts in mind, let’s return to Jewish law (halakha) to see if it is permissible for Jews to partake in the holiday. To answer this question, we should turn our attention to a discussion by the important 16th century sage Rabbi Moses Isserles of Poland who in his Yoreh De’ah (178) sets down a rule on whether or not a Jew can partake in tradition affiliated with an idolatrous religious practice - such as the question of Valentine's Day.
Isserles posits that if he practice is beneficial and logical, not done just out of religious decree, and widely followed independent of religious practice, then it is permissible to partake - as long as it is consistent with the Jewish faith.
The act of giving chocolates and sending love letters is surely beneficial and logical and has nothing to do with any religious decree in origin - rather it is rooted in the ideals of courtly love and modern commercialism and as such, would seem permissible.
Rabbi Isserles' line of reasoning is followed up by Rabbi Moshe Feinstein, an extremely important adjudicator of Jewish law, who lived in New York City after emigrating from the Soviet Union in 1936.
Feinstein explains in a responsa on whether one can eat with one’s head uncovered that “It is also clear that if idolaters make it an idolatrous law to eat something of the proper and tasty foods, this would not make it forbidden. And thus for any other pleasurable thing in the world, it shouldn’t be forbidden just because it was made a law by idolaters.” (My own translation from the Hebrew of Igros Moshe, Yoreh Deah 4:11)
But then there is the problem of outward appearances. Should Jews be seen partaking in practices associated with idolaters’ holidays? Rabbi Feinstein didn’t refer to Valentine's Day itself, but he did discuss Thanksgiving and New Year’s Day, both of which began as religious holidays (one Christian the other Roman): “Regarding the question of celebrating on gentiles' holidays out of their beliefs, if it is for the purpose of the holiday then it isn’t permitted by law and if it isn’t with this purpose then it should be banned for reason of outward impression... And their first day of the year and Thanksgiving don’t ban out of law but those who are extra careful may be more strict.” (Even HaEzer 2:13)
As this quote makes clear, while Rabbi Feinstein bans the celebration of Christian holidays, he doesn’t ban the celebration of Thanksgiving and New Year’s, though he does say that stricter adherents of Jewish law should forgo these. Since Valentine's Day is even less of a religious holiday than these holidays, it seems to this writer that Jews should be permitted to celebrate, as long as Valentine of Rome isn’t venerated – but even the Pope doesn’t do that.