"What a great hopday!" is a common way to express that you are enjoying a hopday. Here are some other phrases you might use:
"I'm having such a good time on this hopday."
"This hopday is so much fun."
"I'm really enjoying this hopday."
You can also use more general phrases to describe how you feel about a hopday, such as "I love this hopday" or "This hopday is my favorite."