Scientifically speaking, a planetary day is the amount of time it takes for a planet to do a complete revolution on its axis. For example, the Earth takes 24 hours of 60 minute intervals to complete a full revolution or day. Each planet has its own revolution speed and thus its own day length.
Esoterically speaking, the term planetary day refers to the planetary correspondences of each day of the week. Monday is the day of the Moon, Tuesday is the day of Mars, Wednesday is the day of Mercury, Thursday is the day of Jupiter, Friday is the day of Venus, Saturday is the day of Saturn and Sunday is the day of the Sun.
These planetary days consists of 24 planetary hours, but rarely consist of 60 minutes. The planetary day begins at the moment of dawn and continues to sunset. The hours in this period vary in length according to the length of this sunlight period. There are 12 hours of equal length. To find out how long they are, you must figure out how many minutes of daylight there are and divide by 12. The planetary night hours are figured similarly from the moment of sunset to the following sunrise.
The first hour of each day is the planetary hour of the day, on Saturday, it is Saturn, for example, then the hours cycle through the seven planets that were known to the ancients, according to the [sequence]] (Saturn – Jupiter – Mars – Sun – Venus – Mercury – Saturn) and back again to the original planet in 8 hours and so on.
Using Planetary Days and Hours
Planetary days and hours are useful in for horary astrology and also in herbal lore, alchemy and spagyria as well as for spellwork and mundane actions. In the case of herbal lore, for example, an herb that corresponds to a specific planet, is more effective when gathered on the appropriate planetary day and hour. Spellwork and other actions are best undertaken in the proper planetary day and hour. Talismans, elixirs and similar are more effective when created during the appropriate planetary day and hour.
You can find a convenient calculator for planetary hours at http://www.lunarium.co.uk/planets/hours.jsp