Khmer (Cambodian) New Years is just around the corner. April 13-15th (and it really starts a couple weeks ahead of time). The beach will be full for a week, mostly with tourists from all over Cambodia, plus the locals take a vacation as well. Best to call ahead and make reservations, between the 10th and 20th of April. More free rooms are "sold" during this period, than any other. Everyone's sleeping on the beach, 'cause all the rooms are sold out. And that's at double or triple price.
You'll see people dancing in the street for New Years. (especially before midnight) and you'll probably get wet, as the tradition is to throw water or bags of water at people (outside). Everyone gets dressed up and goes to the Wats (Buddhist Temples) at least once during the New Year, to celebrate. It's the biggest Cambodian holiday of the year. If you're looking for rooms, look to Victory Hill, downtown, and around beach road near Serendipity Beach.
For more info, check out SihanoukVille Backpacker website.
It's a fun time here.... Come see for yourself!