Weekdays are the best time to visit the Ancient City to avoid the crowds and explore the museum at a leisurely pace. Weekends and public holidays can get crowded with locals and tourists, so plan your visit accordingly. Thailand is a tropical country so the days can be quite hot and humid; plan your visit for early mornings or evenings to avoid the midday heat.
Winter months, between November and February, is when the weather is the best. The cooler days make it easier to go on outdoor adventures. This is the perfect time to visit Ancient Siam, although there may be a larger influx of tourists. If you want to avoid the crowds, visit between July and September, aka the low season. The only drawback is the sudden rains during this time, but flights and accommodations will be cheaper during this time.