I’m unsure too 
but maybe something like 運動のため[の/が/は]いい日 ? 
Well, that’s my first guess, google translate suggests 運動するには良い日 with には meaning for the purpose of:
https://jlptsensei.com/learn-japanese-grammar/には-ni-wa-meaning/
I hope this helps a little.
Cross-posting your question in the following thread is likely to get you more qualified answers than I can provide though 