When Job's three friends observed Job after his second test from Satan, Job 2 v13 says "then they sat on the ground with Job for seven days and seven nights". At the end of the book God is angry with Job's three friends and in Chapter 42 v8 God commands the three friends "to take seven bulls and seven rams and go to my servant Job and sacrifice a burnt offering for yourselves". Does this mean that 1 bull is for 1 day and 1 ram is for 1 night which covers the whole 7 days and 7 nights -- a bull for each day and a ram for each night they sat with Job and spoke incorrectly?
First, I find the behavior of Job's "comforters" in 2:11-13 at least somewhat commendable. Too often when a friend experiences a traumatic loss we are too quick to offer words of comfort, usually in the form of clichés and inadequately thought out words and phrases that at best do little to provide comfort and at worse make things worse! Job's friends at least respected his feelings and the sensitive nature of his situation, and they simply kept silent. (They initially, however, "raised their voices and wept" when they saw how bad off Job was.)
Second, I never thought the seven days of silence were anything other than seven days of silence, with the comforters' proffered advice coming after the seven days of silence.
Third, though not a big fan of numerology, I think it's safe to say the number seven in Scripture has a special significance. Some call it the number of perfection, as in the number of days of God's creative work combined with one day of rest (not that YHWH needed to recuperate; the Sabbath, after all, was made for man and not vice versa).
The thought does occur to me, however, that rather than "perfection," the seven days in Job chapter two may have had a cultural component. In the Middle East to this very day, for example, the period of mourning and the wearing of clothes of mourning by (say) a widow can be quite protracted. Perhaps, then, the seven days of silent mourning was a cultural norm in Job's day. That seems to have been the case in the death of Israel, Joseph's father (see Ge 50:15), and in the death of Saul and his sons (see 1 Sa 31:13).
As to the significance of the two sevens in 42:8, perhaps they speak of the completeness--or sufficiency--of the sacrifice to avert God's wrath against the counselors' (four minus one) folly. Job's prayer for them may have been, in fact, more efficacious than the sacrifices of bulls and rams!
That the mourning took place on the ground has some symbolic significance, if only that death is a very humbling thing, bringing us low. Job, having lost all his children through sudden death, and his health through the affliction of boils from head to toe, was brought down into the dust, literally and figuratively, and his friends joined him there. Though the text does not tell us, Job's comforters may also have fasted during the seven days, rising only to relieve themselves and lying down only to sleep.