This is a colour slide of a family group sitting at a rather elegant picnic set up, with a little table to hold the food. Obviously, it is virtually impossible to tell who the people are, but I would hazard a guess at the relationships – mother and two daughters, grandmother and (behind the camera and unseen) father.
There is no date on the slide, which is a prinzcolour, but the clothes being worn, and the thermos flask design suggests the 1960s or possibly 1970s to me.
As for location without any landmarks it is difficult to pin it down unless you actually recognise it. The other families in the background suggest that is is a regular picnic spot, and the fact that this family has brought a table implies that they probably drove there so it would be a park or common with a car park.