Google Calendar: recurring 20th monthly while working days

I wanted to make recurring ‘workday 20th’ monthly, but this code sometimes hits 19th working day even if 20th is a working day too.(sometimes hits 18th as well) I have no idea to fix this.

BEGIN:VCALENDAR
VERSION:2.0
BEGIN:VEVENT
RRULE:FREQ=MONTHLY;INTERVAL=1;BYSETPOS=14;BYDAY=MO,TU,WE,TH,FR
SUMMARY:(title)
DTSTART;VALUE=DATE:20200101
SEQUENCE:0
DESCRIPTION:(description)
END:VEVENT
END:VCALENDAR