From manual:
override (read/write)

This attribute specifies a list of actions and dates overriding the actions specified in the action attribute. The format of the override specification is action date. action must be one of "full," "incr,""skip," or one of the numbers 1 through 9. date must be either a fixed date or recurring date. Fixed date is of the form "month/day/year." Month and day are 2 digit numbers, year may be either 2 or 4 digits. If the year is 2 digits, numbers in the range 70-99 are assumed to be offsets from 1900, those in the range 00-69 are assumed to be offset from 2000. Recurring date is of the form ¿[number] weekday every [ number] period¿. number can be a number (1, 2, 3, etc.) or an ordinal (first, second, third, etc.), and it is optional. weekday must be one of "monday," "tuesday," "wednesday," "thursday," "friday," "saturday," "sunday." The use of "last" for number is not supported (for example, "last friday" cannot be used to refer to the last Friday of the month). period must be one of "week," "month," "quarter," or "year." Action/date pairs are separated by commas (¿,¿).

Example: override: full 1/1/1994, full first friday every 2 week;

I think help option on the NMC GUI itself should help.