As in previous years, we are including the 2001 Epidemiological Calendar for our readers' practical use and easy reference. The epidemiological calendar is comprised of 52 epidemiological weeks that divide the 365 days of the calendar year. Its use during surveillance activities is important because by standardizing the time variable, it provides a means to compare epidemiological events that occur in a country during a specific period of time to others occuring at a later time or in another country.

The 2001 epidemiological calendar actually begins on the 31st of December 2000. This is due to the fact that the first epidemiological week of the year ends, by definition, on the first Saturday of January, as long as it falls at least four days into the month. January 6, which meets this requirement, should therefore end the first epidemiological week of the year 2001. Each epidemiological week begins on a Sunday and end on a Saturday.