A household's threshold is artificial and spacial. The thresholds of sunset and sunrise are natural and temporal. The thresholds of Solstices have at least one effect in the Dresdenverse with Summer and Winter, and are natural and temporal. So, the question is, do the human institutions of temporal management--weeks, months, years, decades, centuries--also have a threshold effect on magic?
Or, in other words, does year's end have an effect? (and, slightly more snarkily, did the White Council have a Y2K issue, with the end of the millennium on top of everything else?)