This template takes one unnamed parameter. It is designed to prefix a date value with a sortkey using a default year of 2020 if the year is not provided (for use in table cells). It only does this with values of the form Mmm dd (month name, space, day number); input values in other formats are emitted without change. Examples:
'{{Dds20|June 12}}'→ 'June 12''{{Dds20|September 12, 2019}}'→ 'September 12, 2019''{{Dds20|November 2017}}'→ '{{Dds20|November