In ICS Definitive Guide #2 we looked at the scheduling of integrations and mentioned the possibility of using iCal. In this tip we shall explore advanced iCal facility for defining schedules within ICS.
The iCal format is probably best known for sharing meeting events in tools such as outlook. The format is actually a standard defined by the IETF originally in 1998 (https://www.ietf.org/rfc/rfc2445.txt) and updated several times the latest being 
2015 (https://tools.ietf.org/html/rfc7529). The goal of the definition has been for the purpose of driving standardisation across PIM tools (Personal Information Management) tools which includes Outlook and Thunderbird etc. It is a substantial standard the original document ran to 150 pages, but the section supported by the schweduler’s advanced option purely relates to the reoccurence definition covered by section 4.3.10 of the original RFC, and only takes five pages. If you want to know lots about iCal and the varying revisions then you may find https://icalendar.org/ helpful.
It is worth mentioning that today ICS under the hood exploits Oracle’s Enterprise Scheduler Service (ESS). ESS documentation identifies that it works to the 2445 version of the specification (see here). ESS became a 1st class citizen component with the start of the 12c middleware product stack. As a result, if you see any errors processing an iCal configuration the error codes will reference ESS does.