The Torah reading for Shabbos Chol Hamoed Succos and Pesach consists of a reading from parsha Ki Sisa in the book of Shmos. Specifically, it is the text from aliyot three through six of parsha Ki Sisa, but newly divided into seven aliyot for a complete Shabbos reading.
Ki Sisa, you will remember, is when Moshe Rabenu comes down from mount Sinai with the first set of luchos (tablets) and finds that the Jewish people had made a golden calf to worship. Moshe breaks the luchos and G-d punishes the guilty. In addition, G-d declares that He will no longer lead the people to the Promised Land, instead an angel will lead them. Today's reading begins right after that.
In the first aliyah Moshe rabenu asks G-d to reconsider the matter of the angel leading them. G-d reconsiders, and agrees to lead them Himself again.
In the second aliyah Moshe asks G-d, that if Moshe is finding favor in G-d's eyes at this time, can Moshe request to be shown G-d's glory? G-d agrees.
The third aliyah continues the discussion of Moshe asking to see G-d's glory, but here G-d limits what Moshe can see, to the extent that Moshe is able to bear. G-d doesn't show Moshe His face because no one can survive seeing G-d's face, only His back. Rashi says the "back" that Moshe saw was the knot of the tefillin of the head, that G-d wore. Rabbi Shimshon Rapheal Hirsch interprets the "back" to mean that we often can not see Divine providence in something that happens while it is happening, but we can see it later, when looking back at the situation.
In the fourth aliyah G-d tells Moshe to hew new luchos from a sapphire quarry under Moshe's tent that G-d showed Moshe, and G-d says that he will write the words which were on the first luchos on these second set of luchos.
In the fifth aliyah Moshe hews the new set of luchos and goes up Mt. Sinai early in the morning. G-d descends in a cloud and proclaims the thirteen attributes of mercy. G-d says: "G-d - G-d", meaning merciful before the sin as well as after the person repents. "Erech a payim" - "long suffering", meaning slow to anger, G-d waits, perhaps the person will repent. Merciful to the 2000th generation, repaying iniquity to the 4th generation (Rashi points out that the attribute of mercy is 500 times greater than the attribute of punishment). This passage is recited daily in the confessional prayer tachanun (siddur p. 62) when we are asking for mercy for our sins. We also say it many times during Na-ila prayer at the close of Yom Kippur, and also when we take the Torah out of the Aron Hakodesh on Rosh Hashana, Yom Kippur, and when a festival falls on a weekday.
The aliyah continues with G-d saying that He will make a covenant with the Jewish people and do wonders for them; and all the people that we find ourselves in the midst of, will see how awesome is G-d.
The sixth aliyah continues the conversation of G-d and Moshe on Mt. Sinai. G-d here says that he will do wonders for the Jewish people driving out the nations of Canaan before them. G-d then gives instructions to destroy the altars of those nations, and not to make any covenants with them, lest they be a snare to us and intermarry with us.
In the seventh aliyah G-d gives the commandments to observe the three festivals of Pesach, Shavuos, and Succos and to appear in Yirushalayim on those three occasions. Then various additional commandments are given, including not making any molten gods, not to eat chumatz on Pesach, to sanctify the firstborn male (pidyon haben) and cattle, to not work on Shabbos, observation of the festival of Shavous, and finally, not cooking meat and milk together.
An anecdotal summary of the festivals (from Rabbi Shlomo Majeski) is as follows: Pesach is the holiday where you cannot eat WHATEVER you want, but you can eat WHEREVER you want. Succos is the holiday where you can eat WHATEVER you want, but you can't eat WHEREVER you want. And Shavuos is the holiday where you can eat WHATEVER you want, WHEREVER you want, but you can't eat too much because it's only two days long.
The maftir reading for Shabbos Chol Hamoed consists of reading of the services performed in the Bais Hamikdash required for the present day of the current holiday. Because we unfortunately don't have the Bais Hamikdash at the present time, we read about this temple service in the Torah, and we say it as part of the Musaf prayer, as a substitute for the actual service being performed. Let us hope and pray that the Bais Hamikdash is speedily restored to us, so that we can perform this service is actual deed, speedily in our days, amen.