August in Glyfáda, Greece, brings average daily highs of 34°C and overnight lows of 25°C in the 2016–2025 historical record. Allow room to shorten long outdoor plans when the day's forecast is hotter than you want. A schedule with easy breaks and less travelling between stops will give you more choices.
In the record, at least 1 mm of rain or snow falls on 2 of 31 days on average, a daily chance of 5%. Rain has been relatively infrequent in this record, which leaves more scope for an outdoor plan. Still keep an alternative for a wet day: a low monthly average cannot tell you when the rain will arrive.
The calculated day length, sunrise to sunset, averages about 14 hours. Sunshine averages 13 hours a day. The longer daylight window gives you more choice about when to be outside; it does not mean every part of the day will be sunny. Sunshine is a separate measure, and those hours need not arrive in one uninterrupted stretch.
If the coast is part of your plan, check water conditions separately. Air temperature and sunshine do not establish swimming conditions, surf quality or diving visibility. If food is a reason for the trip, leave time around meals rather than filling every gap with another journey. Check the places you choose for current opening hours.
September has an average daily high of 30°C, 4°C lower than August. Its historical rain-day chance is 8%, compared with 5% this month. If your dates are flexible, compare those trade-offs with the kind of days you want. Neither set of averages predicts the conditions of one particular holiday.