Electional astrology, also known as event astrology, is a branch found in most traditions of astrology

Astrology

Astrology consists of a number of belief systems which hold that there is a relationship between astronomical phenomena and events in the human world...

 in which a practitioner decides the most appropriate time for an event based on the astrological auspiciousness of that time. It is distinct from horary astrology

Horary astrology

Horary astrology is an ancient branch of horoscopic astrology by which an astrologer attempts to answer a question by constructing a horoscope for the exact time at which the question was received and understood by the astrologer...

 because, while horary astrologers seek to find the answer to a question based on the time the question was asked, electional astrologers seek to find a period of time which will result in the most preferable outcome for the event being planned.

Historically being used primarily to plan battles, electional astrology has been used by its proponents to plan a number of events, including weddings and trips.
A rudimentary form of electional astrology was used by the Babylonians beginning sometime after the 16th century BCE.