Oracle Scheduler - Oracle 10g/11g Data and Database Management Utilities - page 205

Interval clause : The interval clause defines the time when the next occurrence of the
schedule will take place; it ranges from 1 to 999 and its default value is 1.

Let's assume this scenario; if the user defines a schedule to occur HOURLY at an
interval of three (hours, it uses the same unit as the frequency clause), starting at
midnight with no ending clause defined, it means that the job will first run at 00:00
hours, and the next occurrence will be at 03:00 hours, next will be at 06:00, next will
be at 09:00, and so on. In this example the time expression is:

'FREQ=HOURLY;INTERVAL=3'

Timing Specification: : The individual timing clauses that define a timing expression
are detailed in the table below.

BY{ * } clause Syntax and Definition

BYMONTH BYMONTH = { ( JAN, FEB, ... DEC ) | ( 1, 2, ...

12) }
This specifies on which month or months the job is scheduled to be
run. The syntax allows this time expression to be written either with
numbers or with the month acronyms. Several months can be specified.
Let's assume a task is scheduled to run in July, October and December;
the time expression should be defined to run yearly, on the previously
specified months, and its syntax would be:
FREQ=YEARLY;BYMONTH=JUL,OCT,DEC

BYWEEKNO BYWEEKNO = {1, 2, ... 53}

This defines the Week number according to the ISO-8601 standard.
A week starts on Monday and ends on Sunday, its value ranges from
1 to 52 (or 53 in a leap year). Parts of a week can be found on the
previous year and parts of a week may be found on the next year.
The BYWEEKNO is valid only on a YEARLY interval. According to
the standard the week containing the first Thursday of the year is
considered the week number one, for example on January 2004, the
first day of the year was on Thursday, so this was considered the first
week of the year, and it started on Monday 29th December 2003.