July to August for a couple of weeks depending on the weather, can vary a surprising amount depending on where you are in the UK.
Beautiful deer and rather inquisitive as far as deer go.
If you are walking in the woods and you bump one, go straight down on a knee behind some cover if you can manage it.
Roe will usually run to the next significant obstacle and stop broadside to have a better look at you. If you want to get a good look or a photo at them find a patch of grassy land surrounded as much as possible by trees, get to a place where your wind isn't blowing into the field or wood where they're coming from and wait out last light.
During the rut you will see them out at other times, the bucks hang their tongues out of their mouth and trot in a characteristic gait.