RFC 5545, "Internet Calendaring and Scheduling Core Object Specification (iCalendar)", September 2009

Note: This RFC has been updated by RFC 5546, RFC 6868, RFC 7529, RFC 7953, RFC 7986, RFC 9073, RFC 9074, RFC 9253

Errata ID: 3779
Status: Verified
Type: Technical
Publication Format(s) : TEXT
Reported By: Daniele Mancini
Date Reported: 2013-10-31
Verifier Name: Barry Leiba
Date Verified: 2013-10-31

Section 3.3.10 says:

represents the last Monday of the month.  The numeric value in a
BYDAY rule part with the FREQ rule part set to YEARLY corresponds
to an offset within the month when the BYMONTH rule part is
present, and corresponds to an offset within the year when the
BYWEEKNO or BYMONTH rule parts are present.  If an integer
modifier is not present, it means all days of this type within the
specified frequency.  For example, within a MONTHLY rule, MO
represents all Mondays within the month.  The BYDAY rule part MUST
NOT be specified with a numeric value when the FREQ rule part is
not set to MONTHLY or YEARLY.  Furthermore, the BYDAY rule part
MUST NOT be specified with a numeric value with the FREQ rule part
set to YEARLY when the BYWEEKNO rule part is specified.

It should say:

represents the last Monday of the month.  The numeric value in a
BYDAY rule part with the FREQ rule part set to YEARLY corresponds
to an offset within the month when the BYMONTH rule part is
present, and corresponds to an offset within the year when the
BYMONTH rule part is not present.  If an integer
modifier is not present, it means all days of this type within the
specified frequency.  For example, within a MONTHLY rule, MO
represents all Mondays within the month.  The BYDAY rule part MUST
NOT be specified with a numeric value when the FREQ rule part is
not set to MONTHLY or YEARLY.  Furthermore, the BYDAY rule part
MUST NOT be specified with a numeric value with the FREQ rule part
set to YEARLY when the BYWEEKNO rule part is specified.

Notes:

Other than the missing 'not', pointed out in the errata 1913, the original text contradicts itself regarding BYWEEKNO.
At the beggining of the paragraph, it says that when using a YEARLY frequency with BYWEEKNO rule part, the integer part identifies an offset within the year (i.e. either in [-53, 0) or (0, 53], as it can be a week day in a valid week of the year, even if it is not clearly specified), but at the end of the paragraph, it states that offsets should not be specified in conjunction with a BYWEEKNO rule part.

Report New Errata