Commentaries:
About a month before His own death and resurrection, Jesus visited Bethany and performed His third miracle of resurrection, raising Lazarus from the dead (John 11:1-46). No one knows how often Jesus visited the home of the sisters, Mary and Martha, and their brother Lazarus, but Scripture records some of His visits to their friendly, peaceful, and loving home (Matthew 21:17; Mark 11:11, 19; Luke 10:41-42).
This resurrection is the most extraordinary of all His great works while in the flesh. It foreshadowed His own resurrection, made a profound impression in Jerusalem, and in contrast, brought the wrath of the Sanhedrin to a head, stirring them to decide to murder Jesus. After performing this miracle, He withdrew to the wilderness of Ephraim for some private time with His disciples before the Passover and His final hours.
Martin G. Collins
The Miracles of Jesus Christ: The Resurrection of Lazarus (Part One)
The plain meaning is that He recognized the twelve hours of daylight to form a "day," and the corresponding twelve hours of darkness He called "night." Thus three days and three nights would be made up of six twelve-hour periods or 72 hours.
This is the same method He uses in the Old Testament—in fact, in the very first chapter of the Bible! "God called the light Day, and the darkness He called Night. So the evening and the morning were the first day. . . . So the evening and the morning were the second day. . . . So the evening and the morning were the third day" (Genesis 1:5, 8, 13). Here are three days and three nights so clearly defined anyone can understand.
Richard T. Ritenbaugh
'After Three Days'
With Lazarus' death imminent, Christ's friends were to learn that the wisdom of godly love does not always shield its recipients from suffering, sorrow, and death (John 16:20-22; II Corinthians 7:9-10). Even the personal affection that His friends enjoyed with Him did not persuade Jesus to stray from His responsibility to glorify God in all that He did. So the family had to experience illness and grief.
The gospels do not record the nature of Lazarus' illness, but it was serious enough for his sisters to request Christ's intervention, expecting Him to immediately heal the disease. The ease and simplicity of their message, “Lord, behold, he whom You love is sick” (John 11:3), shows the faith they had in His ability to heal.
Martin G. Collins
The Miracles of Jesus Christ: The Resurrection of Lazarus (Part One)
Other Forerunner Commentary entries containing John 11:9:
Matthew 12:38-40
John 11:1-46