I've seen some folks referring to this Shabbos as Shabbos ha'Gadol. Technically, next Shabbos is Shabbos haGadol, but the MB (430:2) writes that the Rav should give his derasha this Shabbos. If the Rav starts teaching hilchos pesach in a derasha next Shabbos, it's too late to make a difference.
The Rama writes that the minhag is to read the haggadah mincha time on Shabbos haGadol. GR"A quoted in Biur Halacha objects. We read in the haggadah יכול מר״ח...בּעבור זה- בּשעה שׁמצה ומרור מונחים לפניך . On Shabbos haGadol there is no מצה ומרור מונחים לפניך. The question is, "So what?" Of course you can't fulfill the mitzvah of haggadah on Shabbos haGadol because it's not the time for matzah and maror, but that's not the intent. It's just a minhag b'alma, to familiarize oneself with the text beforehand. Why does the GR"A read the derasha as a *prohibition*?
My SIL Aryeh sent me a mareh makom to a Yaavetz who suggests there may be a problem of bal tosif here. I don't understand how that can be. Bal tosif does not apply outside the zman ha'mitzvah when one has no kavanah for a kiyum mitzah (see R"H 28). Also, if that was the GR"As intent, he should say "bal tosif."
Perhaps one can explain the GR"A based on a Terumas haDeshen quoted in Rama 471:2 (in the MB it says in parenthesis next to this din that it is in Th"D siman 7. I'll save you a lot of time. It's in siman 125 here.) that says if a katan is too little to really understand what is going on on pesach night you can feed him early. However, if the katan is within chinuch age and understands, you should make him wait and not give him matzah too eat early. Why? Explains the Th"D: כיון שׁאין המצה חידושׁ הוא לקטן. If he eats matzah early, what he sees at the seder is old news. There is nothing striking that will elicit his curiosity. (See as well the Th"D in 137 regarding starting the seder before nighfall: והשׁינוי נמי אין כּאן when the right time is reached.)
We see from the Th"D that eating matzah early is not just a chisaron in the kiyum of matzah, like Rashi writes in the beginning of Arvei Pesachim (99b) that you can't eat matzah on erev Pesach from mincha time onward כדי שיאכל מצה של מצוה לתיאבון, but it's a chisaron in the kiyum of sipur and haggadah because it turns the experience into something stale. The sipur also has to be לתיאבון.
I don't want to repeat my entire post from 5 years ago, so I'll just give you the punchline from the Ramban at the end of P' Mishpatim (24:1): ולא יאמר ׳ויספר׳ אלא בחדשות אשר יגיד. The word "sipur" means you are relating something new. Old news doesn't count. I wanted to suggest that based on this Ramban we have a new insight into what the goal of "sipur" yetzi'as Mitzrayim is. It's not just to tell the story of what happened, but it is to make that story fresh, exciting, relevant – something new.
R' Akiva Medlov suggests that this is pshat in the GR"A as well. To come back to the Th"D:
דאין להאכילו מצה דדרשינן והגדת לבנך כו' ובעבור זה לא אמרתי אלא בשעה שיש מצה ומרור מונחים לפניך ואם ימלא הבן כבר כריסו מן המצות היאך שייך לומר בעבור זה כיון דאין המצה חידוש לו לקטן:
Leil ha'seder is the birthday of the Jewish people. If you got a peek at your birthday present in advance, it spoils the surprise when you open the box; kal v'chomer if you got a slice of birthday cake in advance, it's just not the same. Matzah is our birthday cake or birthday present; sipur is like singing "Happy Birthday" at the party. If you have a slice of matzah in advance and sing the song beforehand, whether it is the afternoon beforehand or the Shabbos haGadol beforehand, the party isn't much of a party. The cake is not as fresh, the excitement of opening the presents and finding out what you got isn't there That's what bothered the GR"A. Don't ruin the party. Don't ruin the freshness and excitement of leil ha'seder.
Even if you follow the Rama and not the GR"A, that's a goal to keep in mind.