Mid-afternoon is not a precise measure of time but is rather a vague term to refer to the approximate middle of the time between 12 p.m. and sunset. Because the sun sets at different times throughout the year and at different latitudes across the planet, there is no one precise time that can be defined as mid-afternoon.
If an old lady asks her maid to clean the kitchen in the mid-afternoon, a general guideline is being set that means the maid should clean the kitchen at an approximate midpoint between noon and sunset. If "afternoon" time is considered to last from about 12:01 p.m. to 6 p.m., for example, mid-afternoon would be around 3 p.m.
