Day progressed up the levels since being plucked out of local league football by former Boro boss Steve Kittrick in 2016.
Manager Jono Greening said: "Firstly, I want to say a big thank you to the Boro fans for backing the Boost The Boro campaign and raising funds. Their help has made this possible.
"We are absolutely delighted to have Jake join us. He is a player we have been tracking for a while now. He is somebody who wants to play for us and can help us achieve our goal of promotion.
"Jake is an honest, hard-working striker, who is a team player and a goalscorer.
"He wants to continue to learn and to improve. Myself and the rest of the staff are really looking forward to working with him."
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