December in Mueang Nonthaburi, Thailand, brings average daily highs of 31°C and overnight lows of 23°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 8%. 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 11 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 32°C, 1°C higher than December. Its historical rain-day chance is 11%, compared with 8% 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.