X12 -Arima and similar programs cover only monthly and quarterly data. As there are 52 weeks plus one day in a year, weekly data are not strictly periodic.  Cleveland and Scott (2007), Seasonal Adjustment of Weekly Time Series with Application to Unemployment Insurance Claims and Steel Production, Journal of Official Statistics, Vol.23, No.2, 2007. pp. 209–221 deals with the seasonal adjustment of weekly time series.