The Four Shabbatot
As an introduction to this topic, a (relatively) short introductory note would be helpful. On a regular Shabbat, the Parshah of the Week, which is divided into seven portions, is read publicly in the shul, or synagogue. For each of the seven portions, one congregant is "called up" to make "brachot," or blessings, on the Torah, before and after that portion is read, in general by a designated reader. Incidentally, this being called up is referred to as an "Aliyah," as is the act of moving one's residence to Israel, because Eretz Yisrael is considered spiritually higher than anywhere in the Diaspora.
After these seven "aliyot," the "Maftir" Section is read. On a typical Shabbat, this is a short section from the end of the Parshah of the Week, which is repeated. The one called up for Maftir, makes the brachot before and after the short Torah reading and afterwards, recites additional brachot before and after a selection from the Prophets, which he, in general, or the designated reader, in some congregations, reads.
On a Festival, Rosh Chodesh, Chanukah, or Purim or on a Shabbat coinciding with one of the above, Maftir takes on a somewhat different meaning. On those days, it consists of a section relating to the particular Festival or whatever, located at another place in the Torah; hence, on those days, another Torah scroll needs to be taken out.
There are four Shabbatot (Sabbath days) during the year which do not coincide with either a Festival, or Rosh Chodesh, or Chanukah, or Purim, but on which nevertheless, two Torah portions are read, and hence two Torah scrolls are taken out. And if one of these also coincides with Rosh Chodesh, three Torah scrolls are taken out for the reading of three portions.
During these four Shabbatot the regular portion of the week is read and seven persons are called to the Torah. The Maftir is taken from another portion and relates to the theme of the respective special Shabbat, as does the Prophetic portion. The portions read for Maftir during these Shabbatot are as follows: