The speed of sound depends on the temperature. As the temperature of the atmosphere increases, the speed of sound also increases. The time to distinguish between different sounds by the human ear is 0.1second. If we keep the source and the reflector at the minimum distance for echo, the reflected sound will reach us early, in a hot day. So, we will not be able to distinguish it as an echo. But, remember, if the source and reflector are at sufficient distance (say, two times the minimum distance), then we can still hear the echo on a hot day.
The speed of sound increases with increase in temperature. So, clear distinguishable echoes can be heard on cold days, if the source and reflector are nearer. However, if the separation between the source and reflector is very high, for example, if we are shouting from a valley, we can hear the echoes clearly on both cold and hot days.