"And you shall count unto you from the morrow after the sabbath, from the day that you brought the sheaf of the wave offering; seven sabbaths shall be complete: Even unto the morrow after the seventh sabbath shall ye number fifty days; and ye shall offer a new meat offering unto the LORD." (Lev 23:15,16) The Hebrew reads "mimakharat hashabat miyom haviachem et-omer hat'nupah sheva shabatot ...". Litterally, "from the day after the Sabbath from the day you bring the sheaf of the wave offering, seven Sabbath’s perfect they shall be".
The real question is, on which day do we begin counting? The traditional view is that we are to count seven times seven days (Sabbaths of days) from the first day of Chag hamatza (feast of unleavened bread). This means that Shavuot would always fall on Sivan 6. So why would G-d use a method so confusing when all the other Holy Days were on set calander dates as was clearly stated?
Saducee / Messianic tradition says we are to count from the weekly Sabbath following the first Holy Day of the feast of unleavened bread. This means that Shavuot will always fall on the first day of the week but not a set day of the month.