Rahu Kalam and the inauspicious windows
Rahu Kalam, Yamaganda and Gulika Kalam are three daily windows, each one eighth of the daylight hours, that the weekday assigns to different segments of the day and that muhurta practice classically avoids for starting new undertakings.
Daylight cut into eight equal parts
None of these three windows is tied to a planet's actual sky position — despite the names, they are a purely time-based convention. The span from local sunrise to local sunset is divided into eight equal parts, and for each weekday a classical table names which of the eight parts is Rahu Kalam, which is Yamaganda, and which is Gulika Kalam. Because daylight length itself varies with the seasons and the latitude, the clock time of each window shifts from day to day even though its position within the eight parts, for a given weekday, does not.
All three windows depend on knowing the actual local sunrise and sunset for the place in question, computed astronomically rather than assumed — which is also why none of the three can be read at latitudes where the sun does not rise or set on the day in question.
Rahu Kalam
Rahu Kalam is the best known of the three and the one most consistently avoided in everyday practice: which eighth of the daylight span it falls in depends on the weekday, running from the second part on a Monday down through the eighth part on a Sunday. During the window it names, the classical guidance is to avoid beginning new undertakings; it does not describe any quality of the graha Rahu itself, only the time segment named for it.
On a Mangalavara (Tuesday), Rahu Kalam is classically the seventh of the eight daylight parts — so for a sunrise-to-sunset span of twelve hours, it falls in the ninety-minute stretch beginning ten and a half hours after sunrise.