NEW YORK — Some organizers of Juneteenth celebrations around the city admit they’ve been learning about the origins of the commemoration, but are eager to bring New Yorkers up to speed on the nation’s newest federal holiday. President Joe Biden signed a bill last year designating Juneteenth a federal holiday, and its second official celebration will be marked nationwide on Monday. But many events are sticking to the actual date and will roll out Sunday. “I didn’t know anything about it at first,” Brooklyn organizer Athenia Rodney, who presides over the Juneteenth NY Festival — now in its 13th …