The ISO calendar date consists of three parts: year (almost the same as Gregorian year), week number (1-53) and day within a week (1-7). The first week of a year is the week of the first Thursday. The last week is the week immediately before the first week of the next year. Every week starts on Monday. So ISO calendar new year may start 1-3 days before Gregorian new year or 1-3 days later. E.g. a new 2020 year by ISO calendar starts 2 days before, on December 30.
The calculator below converts ISO date to Gregorian date.
The following calculator converts Gregorian date to ISO date.