May in Acharnés, Greece, brings average daily highs of 25°C and overnight lows of 16°C in the 2016–2025 historical record. Light clothing is a useful starting point, with room in the plan to pause indoors. Decide how much walking to fit into each day using the local forecast, rather than treating the monthly average as a limit.
In the record, at least 1 mm of rain or snow falls on 5 of 31 days on average, a daily chance of 17%. Leave some freedom to swap an outdoor day with a sheltered one. There is no need to read the average as a fixed number of lost days; it says nothing about how long rain lasts on each occasion.
The calculated day length, sunrise to sunset, averages about 14 hours. Sunshine averages 12 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.
For nature trips or walks, keep the longest outings movable and check route conditions locally; a monthly climate average cannot establish whether a particular path is accessible.
June has an average daily high of 31°C, 6°C higher than May. Its historical rain-day chance is 14%, compared with 17% 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.