Why does the Torah not insert 'Chag' on Shavu'os like it does by all the other Yamim-Tovim?
Why does the Torah refer to Shavu'os as "Yom ha'Bikurim"?
Rashi: Because on Shavu'os they brought the Sh'tei ha'Lechem, 1 which comprised wheat from the first crops of the year.
Rashi (in Pesachim, 36b): Which permitted Korbanos from the new crops and Bikurim to be brought to the Beis-Hamikdash.
And what are the connotations of "be'Shavu'oseichem"?
Seforno: It means 'on account of 1 the weeks of the harvest season that Hashem kept for you'.
Targum Yonasan: It means 'at the end of the seven-week period (of Sefiras ha'Omer).