Were bikkurim actually brought on Shavuos by the farmers or did they wait until the day after Shavuos (until the wheat bikkurim, mincha chadasha was brought) to start bringing the bikkurim?
Thank you for your question.
The first time that we may bring Bikkurim is on Shavuos, and if they were brought before then, the owner would have to wait until Shavuos to bring them. Bikkurim can be brought on Yom Tov itself, right after the “Shtey Halechem”, (see MInchas Chinuch 91-10) even though it is Yom Tov, the Bikkurim itself can be brought. Regarding the korban brought with it, see the Minchas Chinuch cited above.