Just checked and the event is April 21st.
Long Beach has a Khmer New Years Celebration at El Dorado Park. My Khmer friends say there is tons of good food to be had, but I haven't gone yet. If anyone has or will be going, please let us know how it is.
In Chinatown there is a restaurant called Batambang. I just tried to look it up, but I can't find a listing. It's on New High St north of Caeser Chavez on the east side of the street. I have been there more than a few times and I always get the same things because they're so damn good, I figure "why mess with success?" The first is their beef salad. The salad is a mixtures of S/E asian green and herbs. The beef is enzymed cooked (marinated in lime juice) and tossed in. My wife and I were working in Vietnam for a couple of months, and this dish really takes us back there - more so than any of the Vietnamese restaurant we frequent. The other thing we order is the fishcake cooked in coconut milk. The fishcake has a wonderful note of lemon grass. You could clean this dish up a bit, halve the portion and put it on a nouvelle cuisine menu for three times the price and no one would bat an eyelash. It is delicious!
You also can go the Wad Thai temple in North Hollywood which is big with the Chowhounders. Cambodians and Thais observe the same religious calendar so this temple attracts a fair number of Cambodians. There are plenty of posting about Wad Thai, but on weekends there is essentially a food court on th grounds of the temple with lots of good eats.
My last suggestion is to head to Long Beach which has the largest population of Cambodians outside of Cambodia! Hak Heang is a really fun place to eat. Some of their dishes head a little towards the Chinese, but they will guide you to more authentic Khmer dishes. What really makes this place is that on weekends they have live music which to say the least is kitsh. Here is Jonathan Gold's review:
didn't know about the battambang in Chinatown. there's one on San Gabriel Blvd... just south of Valley Blvd. If you go down to Long Beach there's a large Cambodian community. I've been to a place called Hak Heng (sp?)... large banquet style restaurant with constant Karaoke performances. It's very loud.