December in Acharnés, Greece, brings average daily highs of 15°C and overnight lows of 9°C in the 2016–2025 historical record. Bring layers that work for a longer spell outside as well as a short walk. A warm extra layer gives you options when the day's weather feels cooler than the monthly average suggests.
In the record, at least 1 mm of rain or snow falls on 8 of 31 days on average, a daily chance of 26%. 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 10 hours. Sunshine averages 7 hours a day. Use the daylight window to plan the length of excursions, and check local sunrise and sunset times for the dates of your stay. 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.
January has an average daily high of 13°C, 2°C lower than December. Its historical rain-day chance is 26%, compared with 26% 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.