The corresponding WT explanation goes like:
*** w96 5/15 p. 28 Jonah Learns About Jehovah’s Mercy ***
Jesus was dead and in the tomb for at least a period of time on Nisan 14, was entombed throughout the entire day of Nisan 15, and spent the nighttime hours of Nisan 16 in the tomb. When certain women came to the tomb on Sunday morning, he had already been resurrected.
Jesus was in the tomb for parts of three days. His foes thus got “the sign of Jonah.