1. The Mishnah says that people who are sent to do Mitzvos ("Sheluchei Mitzvah") are exempt from the Mitzvah of Sukah.
2. The Mishnah continues and says that sick people and their attendants are exempt from the Mitzvah of Sukah.
3. In general, one who is involved with one Mitzvah is exempt at that time from another Mitzvah.
4. A mourner is exempt from wearing Tefilin on the first day of his mourning.
5. There is a dispute about how the people who wanted to perform the Avodah of the Korban Pesach became Tamei.
A BIT MORE
1. This exemption applies even when they are resting for the night on the way to their goal.
2. The Mishnah concludes that one is allowed to eat in a temporary manner outside of the Sukah during Sukos.
3. Additionally, one who is preoccupied with performing a Mitzvah is exempt from other Mitzvos, as opposed to one who is preoccupied with mundane matters who is obligated to focus on performing the Mitzvos at hand.
4. Hashem told Yechezkel that he, as opposed to other mourners, should wear his Tefilin. From this we derive that the exemption is only for one day, as the verse says, "And its aftermath is like a bitter day."
5. Rebbi Yosi ha'Glili: They were the people who carried the coffin of Yosef ha'Tzadik. Rebbi Akiva: They were Mishael and Eltzafan who buried Nadav and Avihu. Rebbi Yitzchak: They were people who had dealt with a Mes Mitzvah.