A week might be too short a period of time to show a trend. If it continues, then something must have changed. Could it be a Quality Score issue or smart pricing issue? Re you sure your ads haven't dropped position?
Yes the holiday may explain it. Was also wondering if this could be due to yahoo (US) changes. Trust yahoo would do loads of advertising in the US now, as they changed the bidding mode, to increase ROI for advertisers.
Can't judge that from here...(Asia). Any yahoo ad campaigns running in the US (TV etc..)?