The Tur & Shulchan Aruch siman 430 says that the Shabbos before
Pesach is called Shabbos Hagadol. The reason is ----.
1) It is very strange that the Tur & S. A. designated a complete siman
informing us why people call this Shabbos ,"Shabbos
Hagadol" . There is not a single Halacha in the whole Siman.
2) Why do they not tell us the reason we call it Shabbos Shira, Shuvo etc.
According to Dikduk this coming Shabbos should be called Shabbos Hagedolah (not Hagadol).
Some say that that Gadol refers to the Nes (Shabbos of the Nes Gadol) or Gadol refers
to the Rav who gives the Drasha. (Shabbos of the Rav)
These are all tirutzim ,it really should be called Shabbos Hagedola. Shabbos Hagadol
Could be, if everybody in Klal Yisroel has a minhag even if it is wrong ,since it is done by
a Rabim in Klal Yisroel it becomes a Halacha in Shulchan Aruch.
This is what the Tur & Shulchan(O. Ch. 430) are trying to teach us. The Shabbos before
Pesach is called Shabbos Hagadol.