It’s the Civil War era, and Mount St. Vincent’s is a military hospital in Central Park!
Note the American flag flying there on the left, atop one of the rambling frame wings that Elizabeth Boyle and the Sisters of Charity added to the old McGowan house (center) after they arrived in 1846.
East Drive and the 102nd Street Transverse Road are out of sight on the far left, behind the building with the flag. We are looking from due south, perhaps about 2pm on a late spring day. My guess is this is 1863. The Army had moved in around December 1862. Lacking nurses, they asked the Sisters of Charity to come back to their old property and help out. Did they commute down from Riverdale or move into their old rooms?
By way of comparison, look at the engraving below, made a year or two earlier. Our vantage point is a couple hundred yards farther back, but the artist gives us more detail. Apparently the spires and steeple were still on the chapel in 1861. Were they taken down only when it was Federalized?
This picture comes from the 1862 Valentine’s Manual. Valentine’s was a fat popular annual of 19th Century New York, mainly enjoyed for its postcard-size illustrations you could cut out and hang on your wall.