Pixiv title: 「う」のつく食べ物を食べて元気になろう (Let's eat dishes that had the letter "U" in order to survive the summer heat!)

References:

• Health Hokkaido: Beef Saturday- The Origin of Eel Day
• Wikipedia: Unagi

And, this year...

• The Straits Times: Japanese Eel faces bleak future
• GREATFOOD: The Reason Behind Eel Price Increasing Every Summer
• Esquire: The End of Unagi - Why You Shouldn't Eat Unagi Sushi