Man, it's tough making sense out of Japanese boxoffice, but no matter how you read the numbers Frozen is opening huge.
For whatever reason, Japanese count weekends as only Saturday and Sunday, that's partly why movies opening over 10m are so rare, they only have 2 day weekends.
Frozen is a bit of an anomaly by opening on a Friday, and to make matters even more confusing the Friday it opened falls on some kind of monthly discount day, where tickets are 40% off.
All that being said, even with the discounted tickets Friday estimates are 2.5-3m and admissions for Saturday are going to be higher than Friday (and these will be full price tickets) and from what I gather Sundays are bigger in Japan than Saturdays.
Right now the floor for the 2 day weekend seems to be 8m, with 10m still in play. Definitely over 10m counting Friday. We could definitely be looking at 100m+ for Japan.