Epi Week creates epidemiological week and year from the calendar date.
An epidemiological week is a standardized method of counting weeks to allow for the comparison of data year after year.
The epidemiological week begins on Sunday and ends on Saturday. The first epidemiological week of the year ends on the first Saturday of January, provided that it falls at least four or more days into the month. Therefore, the first epidemiological week may begin in December of the previous year.