Do airlines schedule departures based on average environmental conditions of departing airport?
In general, high temperatures affect the performance of the aircraft in two major ways:


As the density is lower at higher temperature, the thrust produced by the engine is reduced.
The lift produced is reduced as lift is proportional to the density.


Due to these reasons, an aircraft operating in hot condition will have to either reduce weight for taking of from a fixed runway length or increase the takeoff run for taking off with the same weight. Also the climb performance will be affected at high temperatures.

Due to this, the airlines could've schedule operations in conditions when the temperature is less if they believe that takeoff weight is critical for example, in long range fights or where a set number of passengers is necessary for economical operation. 

However, this problem is mostly critical in case of 'hot and high' airports. In this case, this is most probably for facilitating the arrival of passengers in destinations in the morning. 

For example, the SWISS 296 leaves Dar Es Salaam at 11:20 PM (its connecting flight lands only at 1005 PM) and reaches Zurich at 06:16 CET.