Hmm ... it's an animated film about squirrels. Ok ... I've found the script, but unfortunately I think you need to see the movie, or at least that section of it, to know what is meant.
"Heck of a day" can means it's a great day, a special day, an amazing day, or a terrible day, or a day filled with life's ordinary problems and not too much that's cheerful. The complete sentence would be "It's a heck of a day," but we often shorten it and say in a tone of surprise, resignation, weariness, subdued anger, or just a general observation.