"First Call" is a bugle call with three distinct meanings.
U.S. military use
At a U.S. military installation it is a pre-reveille "courtesy" signal, sounded around 05:50, originally to assemble the trumpeters to deliver the reveille that would be forthcoming at 06:00. Some locations also sound it a few minutes before retreat (lowering the flag at the end of the day). In other military contexts it may be used (e.g. 5 minutes) prior to sounding Assembly for any particular formation.
On ships of the US Navy, First Call is sounded at 0755, five minutes ahead of Morning Colors (raising the national ensign), and 5 minutes before Evening Colors (lowering the national ensign). In the absence of a bugle, the word is passed, "First call, first call to colors." The same ceremony takes place on shore establishments but not on ships underway.
At a horse race, it is a signal that all mounts should be at the paddock exit in order to proceed to the track to begin the post parade. The tune is usually sounded by a bugler five to ten minutes before the scheduled start time of the race. The call serves a similar purpose in dog racing. When used for this purpose, the bugle call is usually referred to as the Call to the Post.