This salad develops a better flavor after resting in the refrigerator for a day.

I've seen the comments. You could try this. "This salad tastes better after leaving it for (at least) a day."

Saying 'tastes worst' is quite negative - it suggests that it always tastes bad, but is worst when it is newly made. Possibly 'This salad doesn't taste as good when it is freshly made'. Or 'The flavour of this salad isn't so good on the first day'.or 'The flavour of this salad has not yet developed on the day it is made'. I didn't think salads usually last that long. I think that people here would be quite concerned about food safely if they are left a while!

I mean another way to say how the salad tastes on the FIRST day (comparing to its taste on the days to come).

Yes, exactly. Is there another way to say this?