Late February and March. Whales are present through the whole season, and a December trip can produce them too, but the character of the encounters changes as the season goes on. By late summer they've been feeding for months, they're relaxed, and they become interactive, approaching Zodiacs and hanging around the boats rather than moving off.
That difference is bigger than it sounds. An early season whale sighting is often a blow on the horizon and a glimpse of a back. A late season encounter can be a humpback that stays with the Zodiac rather than moving off.
Nobody can guarantee it, and some voyages get quiet water. If whales are the thing you most want, though, the back half of the season is where to put your money.