December in San José, Costa Rica, brings average daily highs of 24°C and overnight lows of 17°C in the 2016–2025 historical record. Pack clothes you can layer as conditions change. The average leaves room for both warmer and cooler days, so a flexible walking plan is more useful than deciding every outdoor stop before you arrive.
In the record, at least 1 mm of rain or snow falls on 15 of 31 days on average, a daily chance of 50%. Give wet-weather alternatives a proper place in the itinerary. Arrange outdoor plans so you can move them between days, and pack a rain layer rather than relying on a completely dry run of weather.
The calculated day length, sunrise to sunset, averages about 12 hours. Sunshine averages 10 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 a culture-focused visit, group the places you want to see into manageable areas, with time between stops. Check each venue's opening dates before fixing the order. 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.
January has an average daily high of 23°C, 1°C lower than December. Its historical rain-day chance is 38%, compared with 50% 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.