The real answer is, how do you want to make Vitamin C Serum work for you? Using it night or day, or night and day, has different results. Based on what you want to happen with the Serum, use it when you want.
If the real danger to your skin is the sun itself, apply the Vitamin C Serum in the morning after a face wash, and before makeup. To seal in the Serum and make it more effective, use a moisturizer after applying the Serum. The RawChemistry addition of Vitamin E turns the Serum into a natural sunscreen, so you don’t have to add it afterwards. After it dries, apply your regular makeup.
If you are more focused on facial skin repair, apply the Vitamin C Serum at night after a shower, a facial wash, and toner if you use one. The Vitamin C is an antioxidant, cleaning and repairing the skin after UV ray damage. It stops any damage during the day from continuing even in the evening.
Night and Morning
If you can’t decide, and it will fit in with your regular facial skin care regimen, go for both! Then you will be able to protect your skin from harmful UV rays, and repair the skin damage afterwards.