If you are planning to buy a three-day pass for Angkor, go and buy it the night before - you should be able to get in for sunset that same day as well for no extra charge. Bring a passport photo.
Ticket office, main entance to Angkor Wat
Updates
You no longer need a photo as they have the equipment to take one of you there and then. You don't need passport photos now - digital camera at point of sale. This free sunset works for 1, 3 and 7 day passes, but a new innovation is that 3 day passes can be used for non-consecutive days in a week, and 7 days can be used over a month.
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