Avodah Zarah 6: Pre-Holiday Business

Are the "3 days before the non-Jews' holidays" inclusive of the day of the holiday itself? Or really not? What about Sundays? Other holidays are named explicitly too. Also, what if you did do business with the non-Jews on these days? How temporary must a thing be? How guilty is the one who does this? Plus, Rabbi Yehudah HaNasi, consulting Resh Lakish on what to do with regard to this kind of case.