I'm also curious about that. Surely it can't be the target date for the Harper's Ferry action. As far as I know Harriet Tubman was not involved in that. The Secret Six and other John Brown backers are in the dark about that plan. They know he has something in mind and it could involve violence but he is keeping the details to himself and a few other followers. That will change over the summer. Frederick Douglass will be let in on the plan and invited to participate. Perhaps Tubman was as well.
Oh? Wow... so there were more than just "Secret Six" who knew? Hmmmm…. I wonder, how much they knew & supported?