"Payday March" is a

bugle The bugle is a simple signaling brass instrument with a wide conical bore. It normally has no valves or other pitch-altering devices, and is thus limited to its natural harmonic notes, and pitch is controlled entirely by varying the air a ...

march March is the third month of the year in both the Julian and Gregorian calendars. Its length is 31 days. In the Northern Hemisphere, the meteorological beginning of spring occurs on the first day of March. The March equinox on the 20 or 2 ...

. The first strain, repeated, also serves as "Pay Call", a

bugle call A bugle call is a short tune, originating as a military signal announcing scheduled and certain non-scheduled events on a military installation, battlefield, or ship. Historically, bugles, drums, and other loud musical instruments were used ...

to announce that the troops will be paid. :

References

bands.army.mil
Bugle calls {{music-instrument-stub