Read this excerpt from Harriet Tubman: Conductor on the Underground Railroad.

Unfortunately, the discovery was almost always made on a Sunday. Thus a whole day was lost before the machinery of pursuit could be set in motion. The posters offering rewards for the fugitives could not be printed until Monday. The men who made a living hunting for runaway slaves were out of reach, off in the woods with their dogs and their guns, in pursuit of four-footed game, or they were in camp meetings saying their prayers with their wives and families beside them.

According to the excerpt, why does Tubman plan escapes for late Saturday night or early Sunday morning?

Safe houses on the Underground Railroad operate only on Sundays, when everyone else is working.
Such activities as attending church are common on Sundays, so people are less likely to notice they are missing.
Slave trackers do not work on Sundays, so Tubman’s group has more lead time before people start looking for them.
Tubman and her group know exactly where slave trackers are on Sundays, so they know how to avoid them.