When do plants use carbon dioxide night or day?

Plants use carbon dioxide in the daytime as part of photosynthesis which occurs in the chloroplast. They use the CO2 and make sugars. People breath in oxygen and breath out carbon dioxide, plants do the opposite. They breath in carbon dioxide and breath out oxygen.

One small "technical" detail that should be mentioned. Plants really don't breath, they respire. People breath.

So, at the night, the plant cells respire more than they photosynthesise, so they get rid of more carbon dioxide than oxygen.