Each year on Shabbat Hagadol (the Shabbat just prior to Pesach), our shul rabbi explains that the reason it is called "Shabbat Hagadol" is that it means "The Shabbat of the Gadol" ("The great one"). It is a reference to the "gadol" or the rav of the community or congregation.
My rav explained that originally, the rav delivered a sermon only twice yearly - on Shabbat Hagadol and Shabbat Shuva (Shabbat between Rosh Hashanna and Yom Kippur.) He did not deliver weekly sermons as is quite common in shuls, currently.
My question is when and why did the original custom change?