I am trying to set up a recurrent event for “the Monday preceding the first Thursday of all even months of the year” to use in my Thunderbird calendar.

This means it has to be EITHER

1. the FIRST Monday of an EVEN (Feb, Apr etc) month if the Thursday falls on a date from the 4th to the 7th (Monday’s date 1st to 4th)
   OR
2. the LAST Monday of an ODD month (Jan, Mar etc) if the Thursday falls on a date from the 1st to the 3rd (Monday’s date from 26th to 31st, depending on the number of days in a given month)

I can set up either rule for an event, but cannot see a way to integrate both conditions in one RRULE line, since iCal does only accept the last valid condition in a line, or the second RRULE if one sets two.

Any pointers what I could do, or even an “impossible in iCal, move on with your life” would be much appreciated.

My journey so far:

I use Thunderbird as my calendar app
I have used an SQLite editor to edit the cal_recurrence table in local.sqlite, then checked in the calendar app if the behaviour is as expected, then exported the .ics file to look what happened when it didn’t work as I had hoped.

Condition 1 – this works as expected
1 RRULE:FREQ=MONTHLY;BYMONTH=2,4,6,8,10,12;BYDAY=1MO;BYMONTHDAY=1,2,3,4;

Condition – this works as expected
1 RRULE:FREQ=MONTHLY;BYMONTH=1,3,5,7,9,11;BYDAY=-1MO;BYMONTHDAY=26,27,28,29,30,31;

Conditions 1+2 – only the second line is used in iCal
1 RRULE:FREQ=MONTHLY;BYMONTH=2,4,6,8,10,12;BYDAY=1MO;BYMONTHDAY=1,2,3,4;
2 RRULE:FREQ=MONTHLY;BYMONTH=1,3,5,7,9,11;BYDAY=-1MO;BYMONTHDAY=26,27,28,29,30,31;

Conditions 1+2 – only the part from the second “BYMONTH” expression is used in iCal
1 RRULE:FREQ=MONTHLY;BYMONTH=2,4,6,8,10,12;BYDAY=1MO;BYMONTHDAY=1,2,3,4;BYMONTH=1,3,5,7,9,11;BYDAY=-1MO;BYMONTHDAY=26,27,28,29,30,31;