We read in this week's Torah portion of Bo that G-d gave Moshe and Aharon the following commandment: "This month [Nissan] shall be the first of the months of the year for you" (Sh'mot 12,1). It is from here we learn the laws of how the rabbinical court sanctifies the new months and determines the holidays accordingly. One of these laws, as taught in Tractate Sanhedrin, is that this mitzvah may be performed only in Eretz Yisrael.
