Given the significance of sallying forth with their fleet to attack and engage the British, it seems like that would be the obvious point in time to have the U-Boats prowling the area to harass the British fleet, disrupt formations or logistics, etc.
That was actually the plan!
Additional U-Boats were also sent to patrol and lay mines off of Scapa Flow, the Firth of Forth, and Harwich in the hopes of exacting maximum attrition on the fleet. And to denude them of escorts tied down with subs, allowing for more effective torpedo attacks by German light ships.
However due to the technology of the time the Subs could only remain on station a few weeks, and would be hunted themselves by British forces, while also dealing with poor weather and fog.
So when the subs were deployed the operation was slated to begin the second week of May. But mechanical problems and lagging repairs on German warships forced delays(the HSF could ill afford to make her numbers disadvantage any worse than it absolutely had too). Ultimately it would be another 2 weeks before the fleet sailed.
By then some subs had been sunk, some were forced out of position, some had returned home. But those left did attempt to contribute. The Battlecruisers were spotted sailing, and even fired at by U-32.