No, it cannot ever be a fun day, it might have been a day with a lot of good fun, or one on which you had fun, or even a funny day, although this last is probably something completely different to what the person who talked about a fun day meant!
The other day a journalist working for a respected newspaper said on tv that the effects of the earthquake are ‘heart rendering’. Now I have rendered fat, I once had a house with a rendered exterior, but …
I'm not hot on punctuation but even to me Eat healthy is either an instruction, so I say to myself "No I won't, or I want to say what is healthy and where can I buy it? The one that gets me is a sign saying "Pick your own". I used to pass it every day and my boss used to ask me what I'd scratched each day. Her pet hate was "Plant crossing".