"First, Third, and Fifth Thursdays" isn't something that Google Calendar handles well -- there's no option in a repeating event for putting this into a schedule.
Fortunately, someone smart figured out how to do something using a special calendar-file-format, and we've got an adjusted version that can work for our singings.
Create a new empty file using text editor like Notepad (Windows) or TextEdit (Mac)
Copy and paste the following into the file:
BEGIN:VCALENDAR
BEGIN:VEVENT
DTSTART;VALUE=DATE:20240905
DTEND;VALUE=DATE:20250905
RRULE:FREQ=MONTHLY;INTERVAL=1;BYDAY=1TH,3TH,5TH
DESCRIPTION: Sacred Harp
LOCATION:
SEQUENCE:0
STATUS:CONFIRMED
SUMMARY:Sacred Harp Singing at Steinert
DTSTART:20240905T220000Z
DTEND:20240906T0000000Z
TRANSP:TRANSPARENT
END:VEVENT
END:VCALENDAR
Adjust the starting date and ending date on the third and fourth lines. Right now they're set for September 5 of 2024 and September 5 of 2025. Make sure your starting date is a Thursday.
Adjust the 2nd- and 3rd-to-last lines by changing the dates (the first eight digits on each line) to both match the numbers of your starting date. The remaining digits, after the "T", are the starting and ending times, using Zulu time (roughly: Greenwich Mean Time). You should not need to adjust these.
Save your file with the name SH.ics, on the Desktop. It's important that the file extension (the stuff after the dot) is "ics" and not "txt"!
Open Google Calendar and select the settings icon (the little gear) in the upper right
Select "Import and Export" from the left-hand column in the settings page
Click "Select a file from your computer" and navigate to the Desktop and choose SH.ics
Make sure that just below the file-selection, you've chosen the correct calendar.
Click on the "Import" button and your new event, scheduled for whatever duration you selected, should appear.