This was the first call heard for the beginning of a new day, and was often referred to as First Call. It was used to summon all of the buglers together at one location, usually headquarters. The call was sounded by the Chief Bugler. It was at this "meeting of the buglers" where the assigned calls for the day were given.
Soliders upon hearing this call would know that is was time to wake up and start getting ready for morning roll call.