Perhaps it differs at your hospital but our "Grand Rounds" were once a month which had the invited speaker, but there was a weekly lecture we had to attend as well. Perhaps it was just semantics; we considered Grand Rounds as when an invited lecturer came..weekly lectures with home faculty was not so "grand". 😀
This was true at my large academic residency and my different, but still large academic fellowship program. There was always a Grand Rounds every week, but some weeks it was Urology, sometimes Ob-Gyn, Peds, IM, sometimes PRS, etc.
Neither program, nor my current ones had weekly General Surgery Grand Rounds with invited speakers (who are expensive for programs - to fly someone out, wine and dine them and put them up in a fancy hotel once a week). Then again, those programs were much more about slavery than education.
Suffice it to say that it varies widely.