Why April 25? — Penguin Geek answers this:
On the 25th of April every year, the Adelie penguins in the Antarctic begin their annual northward migration. The Adelies start migrating in the Antarctic fall season and won’t return to their colonies until the following spring.
In true synchronized fashion, the Adelies dive into the frigid waters answering their Nature’s call to migrate. They will swim north for only a few hundred miles (around 600km) and stay among icebergs that are floating about, feasting on krill and other penguin delicacies.
So really, these penguins don’t actually go anywhere when they migrate. They’ll bob around in the ocean and rest on the ice until it’s time to head back home to Antarctica.
Actually, the reason they do this is because the days in Antarctica are becoming darker during this time of year. Adelies do not see too well in the dark so they migrate north so they can hunt for food, otherwise they would starve.
“10 emperor penguin facts for World Penguin Day – in pictures” — The Guardian: