Covers off? Are these plants in cold frames?

If you mean fleece covers, etc, then they should be removed when there's no chance of further frost. It really depends on what the plant is though.

Uncover in the day if nice but cover at night until all danger of frost has passed is what I do

Goodness, I'm surprised they've been covered at all this year, we haven't really had a winter, so I assume your plants must be partially tender, you haven't said what they are. Usual practice is to cover when very cold weather is expected, and uncover as soon as it has passed, then recover as necessary, till winter's over.