first call

"First call" is a bugle call with three distinct meanings.At a horse race, where the tune is also known as the Call to the Post, it is a signal that all mounts should be at the starting gate because the race is about to begin. The tune is usually sounded by a bugler five to 10 minutes before the scheduled start time of the race. The call serves a similar purpose in dog racing.
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With  collateralized mortgage obligation (C.M.O.s), the start of the cash flow cycle for the cash flow window.