December in Paulista, Brazil, brings average daily highs of 31°C and overnight lows of 24°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 13 of 31 days on average, a daily chance of 43%. 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 13 hours. Sunshine averages 12 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.
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.
The following month, January, has a similar average daily high of 31°C. Its historical rain-day chance is 60%, compared with 43% 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.