Albert Barnes Commentary Acts 10:9

"Now on the morrow, as they were on their journey, and drew nigh unto the city, Peter went up upon the housetop to pray, about the sixth hour:" — Acts 10:9 (ASV)

Peter went up, etc. The small room in the second story, or on the roof of the house, was the usual place for retirement and prayer (see Barnes on Matthew 6:6 and Matthew 9:2).

About the sixth hour. About twelve o'clock at noon. The Jews had two stated seasons of prayer—morning and evening.

But it is evident that the more pious Jews frequently added a third season of devotion, probably at noon. Thus David says, Evening and morning, and at noon, will I pray, and cry aloud (Psalms 55:17). Thus Daniel kneeled upon his knees three times a day and prayed (Daniel 6:10, 13).

It was also customary in the early Christian church to offer prayer at the third, sixth, and ninth hours (Clement of Alexandria, as quoted by Doddridge). Christians, however, will not merely have stated seasons for prayer, but they will seize upon moments of leisure, and when their feelings strongly incline them to it, to pray.