Hmong New Year celebrations in Luang Prabang & Lak 52 started on 4 Dec (maybe 3 Dec for the latter) 2013. It lasts for about 10 days, so anyone who is in Laos & interested in seeing the celebrations can catch it there. You'll see people dressed in their finest traditional garb, the ball tossing courtship game, singing, dancing & khaen (traditional bamboo wind instrument) performances, cockfighting, some small food & game stalls, & booths where you can wear Hmong traditional dress & have your photo taken. A few links to photos of what the celebrations are like (in Phonsavanh) in this post: http://laomeow.blogspot.sg/2009/11/hmong-new-year-dates-2009.html
Luang Prabang - there will be celebrations at Ban Khoua Thi Neung, & sometimes also at one of the other Hmong villages on the outskirts of central Luang Prabang town. Many hotel, guesthouse & restaurant staff in Luang Prabang are ethnic Hmong, so you could try asking them for more details. Tuktuk drivers will know how to get to Ban Khoua Thi Neung.
Lak 52 - this town is along HWY 13, 52 KM north of Vientiane capital. Buses & vans from Khua Din market (Talat Sao/morning market) bus station in central Vientiane heading north to Lak 52, Thalath, Kasi & Vang Vieng will pass through it. The celebration site is somewhere on the west of HWY 13 (hidden from the highway) - turn left at the local MoneyGram outlet, & then left again to follow a mostly unpaved road - ask locals for help with directions.
Don't know the dates for this year's celebrations at the other major festival sites in Laos - Phonsavanh, Sayabouly town & the large Hmong village ~22KM north of Pak Mong (along HWY 13 in the direction of Oudomxay) - try asking around.