You may have noticed that a typical Japanese calendar always includes a pattern of Chinese characters written into each date box, the same 6 sets of characters ("Roku You" or 6 days) recurring periodically:
先勝 (Sen-shou / Sen-kachi / Saki-kachi)
Good luck in the morning, bad luck in the afternoon.
"You'll win if you hurry". Making decisions and acting quickly upon those decisions will reward you an unexpected fortune. It is also believed this is especially a good day for court actions and lawsuits.
友引 (Tomo-biki / Yuu-in)
Good luck all day, except at noon.
"The day which pulls a friend to disaster." It is also the day when a match remains undecided or ends in a draw. It is a famous superstition that if a funeral is carried out on this day, friends will be drawn to the world of death. Thus, funerals should not be scheduled on this day.
先負 (Sen-make / Sen-pu / Sen-bu / Saki-make)
Bad luck in the morning, good luck in the afternoon.
"You'll lose if you hurry". It is recommended to avoid gambles and hurried business transactions, to act modestly, and wait for others to take initiatives in action.
Maintaining your tranquility is most advisable on this particular day. Some believe that since the word also means, literally, "it'll bring you a bad fortune if you go out (to somewhere)", they avoid blind marriage meetings (omiai).
Bad luck all day.
The meaning of this day is twofold: "The day when even Buddha would have perished" and, more literally, "Everything is hollow and meaningless". It is also believed that an illness that starts on this day will be prolonged.
Celebrations such as a marriage ceremony are avoided on this day. Many people also avoid moving or opening their business on this day.
大安 (Tai-an / Dai-an)
Good luck all day.
"Everything will proceed well and successfully" or "Everything will be peaceful". This is the ideal day for marriage ceremonies, as well as moving or opening a new business.
赤口 (Sha-kku / Ja-kku / Ja-kkou / Sha-kkou / Sekiguchi)
Bad luck all day, except at noon.
Since red is most often associated with blood, people working with sharp-edged tools (e.g., carpenters and cooks) are particularly advised to take special precautions.
It is most strongly advised not to make any congratulatory ceremonies on this day (especially in the evening). It's good to take special precaution against fire on this day.
Almost all Japanese people are aware of the superstitions that are attached to these. In the modern day and age, nobody pays much attention to them, but they still have a residual effect.
In the same way that a modern Western person may try to avoid having a wedding, catching a plane, or starting a new job on Friday 13th, Japanese tend to take these 六曜 (roku-you) into consideration in scheduling important (including business) occasions.
The exact origin of this "roku-you" calendar is ambiguous. Originally, it may have been no more than dividing the 30 days of a month by the number of fingers - thus, a simple way of keeping track of time, with no special meaning attached.
One theory has it that during China's Han Dynasty (B.C. 202 - A.D. 220), it became a famous philosophy that every aspect of life can be divided into six categories, each with its own meaning. Though doubtful as a theory, the present "roku-you" might have derived from this way of thinking.
"Encyclopedia of Modern Day Explanations of Japanese Calendar" by Yoshiro Okada & Suetada Akutsu, Kashiwa Shobo, 1993