for Khmu i don't know.
for Hmong, might still be a bit too early to know the date - usually after the harvest (Nov) the heads of the various Hmong villages in the area will meet to decide on the dates & the venue. for LPB city, the venue sort of rotates between the 7 (or 9? forgot) Hmong villages but the larger ones (e.g. Ban Khoua Thi Neung, Ban Khok Wa Neua/Tai) can have their own even when it isn't their turn to host. in 2009 the main events (dance/song/cockfighting/etc contests) were on the weekend of 16-17 Dec. good place to ask is at My Library in LPB, plenty of Hmong youth in the know hang out there. many of the hotel/restaurant staff in LPB city are also Hmong, just keep asking around.
btw the largest scale celebrations (with bull fights, but between bulls, not bulls & men) are in Phonsavan. there are also celebrations at KM 52 in Vientiane province, Sayabouly town, the big Hmong village (can't recall name right now) not far west of Pak Mong along the Pak Mong-Oudomxay section of highway 13, etc.
dates differ from place to place. event details spreads through word of mouth, mobile phones & sometimes a banner or a notice stuck on a bus station pillar. & even on the day itself event schedules can change at the very last moment...friends taking part in the contests (& who have done so for years) weren't sure of when & where to turn up sometimes...best to have a flexible schedule.
ok just got news that it's prob around 6th Dec 2010 for Luang Prabang town area.
btw Hmong New Year celebration isn't a 1-day affair but stretches up to 2 weeks...so getting the exact date matters only if you're really interested in the rituals (e.g. chicken sacrifice, putting up of new household altars, etc) that are done on the first day.