Maybe that is their problem.
Because exodus seems clear and with Aaron’s declaration of a chag on the 39th day from Moses on the mount, it seems that Christianity could teach something to Judaism if it is the exact same occurence in Acts 2 of the New Testament..
A little too much Talmud is why Judaism allows Rome to tell them what today is..
As opposed to the calendar in the law and the prophets that tell us , by simple count, it is His 4th work day ..
That too would mess with Judaism and christianity but Daniel the prophet said Rome would be a vessel to change times and laws(Daniel 7) so it should not shock us when we see it.
A believer in ‘Scripture only’ might have said Shavuot is a harvest festival not connected to the giving of the Torah on Sinai.