In this poem, the changing days themselves are made feminine, and these girlish days don't fulfil their promises – it's as if Anna Laetitia Barbauld is commenting, with light irony, on the supposed faithfulness and guilt of women.
See where the falling day
In silence steals away
Behind the western hills withdrawn:
Her fires are quenched, her beauty fled,
While blushes all her face o'erspread,
As conscious she had ill fulfilled
The promise of the dawn.
Another morning soon shall rise,
Another day salute our eyes,
As smiling and as fair as she,
And make as many promises:
But do not thou
The tale believe,
They're sisters all,
And all deceive.