I haven't looked at the numbers today, but doesn't part time employment always start to pick up this time of year.
The calculation does not include seasonal jobs
Total employment and unemployment are higher in some parts of the year than in others. For example, unemployment is higher in January and February, when it is cold in many parts of the country and work in agriculture, construction, and other seasonal industries is curtailed. Also, both employment and unemployment rise every June, when students enter the labor force in search of summer jobs.
The seasonal fluctuations in the number of employed and unemployed persons reflect not only the normal seasonal weather patterns that tend to be repeated year after year, but also the hiring (and layoff) patterns that accompany regular events such as the winter holiday season and the summer vacation season. These variations make it difficult to tell whether month-to-month changes in employment and unemployment are due to normal seasonal patterns or to changing economic conditions. To deal with such problems, a statistical technique called seasonal adjustment is used. This technique uses the past history of the series to identify the seasonal movements and to calculate the size and direction of these movements. A seasonal adjustment factor is then developed and applied to the estimates to eliminate the effects of regular seasonal fluctuations on the data. When a statistical series has been seasonally adjusted, the normal seasonal fluctuations are smoothed out and data for any month can be more meaningfully compared with data from any other month or with an annual average. Many time series that are based on monthly data are seasonally adjusted.
Here is more information on how the percentage is calculated (every time) if people are interested.