iCalendar ICS file with an event on the first and third Thursday of each month

first_and_third_thursday.ics

BEGIN:VCALENDAR
PRODID:-//Google Inc//Google Calendar 70.9054//EN
VERSION:2.0
CALSCALE:GREGORIAN
METHOD:PUBLISH
X-WR-CALNAME:Temp
X-WR-TIMEZONE:America/Los_Angeles
BEGIN:VTIMEZONE
TZID:America/Los_Angeles
X-LIC-LOCATION:America/Los_Angeles
BEGIN:DAYLIGHT
TZOFFSETFROM:-0800
TZOFFSETTO:-0700
TZNAME:PDT
DTSTART:19700308T020000
RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=2SU
END:DAYLIGHT
BEGIN:STANDARD
TZOFFSETFROM:-0700
TZOFFSETTO:-0800
TZNAME:PST
DTSTART:19701101T020000
RRULE:FREQ=YEARLY;BYMONTH=11;BYDAY=1SU
END:STANDARD
END:VTIMEZONE
BEGIN:VEVENT
DTSTART;TZID=America/Los_Angeles:20230202T180000
DTEND;TZID=America/Los_Angeles:20230202T223000
RRULE:FREQ=MONTHLY;BYDAY=1TH,3TH
DTSTAMP:20230130T190203Z
UID:3vt2j24piiunfpgf38oh4bgvl4@google.com
CREATED:20230130T190142Z
DESCRIPTION:
LAST-MODIFIED:20230130T190142Z
LOCATION:
SEQUENCE:0
STATUS:CONFIRMED
SUMMARY:Test
TRANSP:OPAQUE
END:VEVENT
END:VCALENDAR

notes.md

More information at : https://icalendar.org/iCalendar-RFC-5545/3-8-5-3-recurrence-rule.html

The pertinent line is

RRULE:FREQ=MONTHLY;BYDAY=1TH,3TH