Depends a bit on what you're looking to see. The humpbacks we get in SouthEast begin their seasonal migration from Mexico and Hawaii in March and April. Most of the animals we watch in the summers begin arriving by late April, and they're all usually here by mid-May. They don't usually leave the area until well in to the fall - October/November in most years. We do have a moderately sized group of animals that stay year-round... just saw 4 small males frolicking near 5-Finger Lighthouse the other day.
Here in Juneau, we don't get to see the Grey Whales that come by - they stay in open ocean, but you'll encounter them if you're on a Gulf cruise. Similar timeline to the humpbacks.
Hope this helps!
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