Marty explained it.

This was the last night on which his paternal roof would shelter him, the days of grace since it fell into hand having expired; and Giles was taking down his cupboards and bedsteads with a view to an early exit next morning.

His encounter with Mrs.Charmond had cost him dearly.
When they had proceeded a little farther Marty was joined by Grammer Oliver (who was as young as the youngest in such matters), and Grace and Mrs.Melbury went on by themselves till they had arrived at the spot chosen by the village daughters, whose primary intention of keeping their expedition a secret had been quite defeated.

Grace and her step-mother paused by a holly-tree; and at a little distance stood Fitzpiers under the shade of a young oak, intently observing Grace, who was in the full rays of the moon.
He watched her without speaking, and unperceived by any but Marty and Grammer, who had drawn up on the dark side of the same holly which sheltered Mrs.and Miss Melbury on its bright side.

The two former conversed in low tones.
"If they two come up in Wood next Midsummer Night they'll come as one," said Grammer, signifying Fitzpiers and Grace.