From ICAO "The aerodrome reference temperature should be the monthly mean of the daily maximum temperatures for the hottest month of the year (the hottest month being that which has the highest monthly mean temperature). The temperature should be averaged over a period of years."
Since this can be quite performance limiting in hot countries, some operational planning might take a look at temperatures throughout the year to establish the 85% percentile annual temperature (good for tourist resorts where most of the traffic is during the cooler seasons), and more commonly might look at the evening temperatures with operations scheduled for 9pm onwards.