Where today's action begins: (p.4) "...on a sunny day, far out at sea (the Pacific Ocean)." The text says the whales were traveling south and that they travel south during the winter, so it must have been winter or have been close to winter. (p.5)
Where key events happen today: Humphrey's excursion carried him under the Golden Gate Bridge into San Francisco Bay (p.7) and up the Sacramento River (p.10) where he actually got stuck.
Where today's events end: In the Pacific Ocean, as "Humphrey swam under the Golden Gate Bridge and back out to sea" (p.34).