Bournemouth and the Portuguese club are part of owner Bill Foley's Black Knight portfolio.
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Day and William both trained with the senior Moreirense squad before returning to England last weekend.
"There was an opportunity for me and Harold to train with them, so just getting that first team experience," Day told the Daily Echo.
“Obviously a club that's part of the Black Knight group, so that's just another option in my career or anyone else's career that's at Bournemouth.
“So yeah, it's just getting open to more eyes, more people seeing your career, and then obviously, that's only a good thing.”