Efficiently storing irregular/repeating intervals (think calendar/events)
Always look for standards that support your requirements. What is a standard that supports recurring calendar events? ICalendar RRULEs:

http://en.wikipedia.org/wiki/ICalendar

http://www.kanzaki.com/docs/ical/rrule.html

You can either store the rule as plain text and parse it as needed, or use a database schema for them:

SQL Schema:
https://stackoverflow.com/questions/1054201/ical-field-list-for-database-schema-based-on-ical-standard/1397019#1397019

Perl:
http://search.cpan.org/~rfrankel/iCal-Parser-1.16/lib/iCal/Parser.pm

PostgreSQL-specific:
http://svn.expressolivre.org/contrib/davical/dba/rrule_functions-8.1.sql
Performance-wise, you can use Materialized Views to calculate events, say one month in advance and one month prior (if required):

https://stackoverflow.com/questions/4239871/when-building-a-calendar-app-should-i-store-dates-or-recurrence-rules-in-my-dat/10151804#10151804


Additional information : http://www.vertabelo.com/blog/technical-articles/again-and-again-managing-recurring-events-in-a-data-model
This article explains each and every aspect of repeated events. Please read it before coming to any conclusions.