The white campion (Silene latifolia) is sometimes thought to be a bleached red campion but it is, of course, a totally different species. I think the confusion comes from the fact that they can quite often be found growing together and they can hybridise giving a pale pink version. The flowers also look very similar if you disregard the colour.
The botanists who wrote my book look far more closely at these flowers than I do and point out that the flowers of the white campion are slightly larger than the red and the calyx is longer and narrower. The white is also slightly fragrant at night although I do not see how that will help with identification unless you find some by day and then go back by torch light to give them a sniff! Overall, I think the difference in colour is enough for me to work on.
The white campion is far less common than the red, both being found on waysides and field edges. The white campion flowers from May through until October.