Regulations in both armies stated that soldiers would receive pay on the last day of even numbered months (Feb, April, June, etc) for that month and the proceeding month. For example: on August 31 they would be paid for July and August. Sometimes it worked that way. On June 30, 1863, a number of regiments of the Army of the Potomac paused on their march north through Maryland to Gettysburg to receive pay. Very often the paymaster was late, as shown by the pay record of the 97th Pa. Inf.
They were mustered in in September, 1861, at West Chester, Pa. and received pay on:
March 12, 1862, for September - December, 1861
April 18, 1862, for January - February, 1862
April 30, 1862, for March - April, 1862 (The only time in 3 years they were paid according to regulations).
September 15, 1862, for May - June, 1862
February 11, 1863, for September - December, 1862
April 21, 1863, for January - February, 1863
June 23, 1863, for March - April, 1863
July 21, 1863, for May - June, 1863
September 26, 1863, for July - August, 1863
December 8, 1863, for September - October, 1863
February 19, 1864, for November - December, 1863
April 18, 1864, for January - February, 1864
September 1, 1864, for March - June, 1864
They mustered out at West Chester in late September and received all pay owed them.