The pasuk calls the first day of Sukkos “byom harishon.” The medrash questions “It’s not the first day of the month—it is the fifteenth!” and proceeds to answer that it is “rishon b’avonosayhem” (the first of your sins). There are those who suggest a meaning, that people don’t have time to do any aveiros, because they are consumed with doing mitzvos of building their sukkah, and preparing the four species (This year in particular, we can understand the idea of having no time).
To explain this idea further, the Taz (581) suggests that being as one is involved in preparation for a mitzvah, his aveiros aren’t counted. However, he asks if that is true, certainly Sukkos itself, when one is involved in doing the actual mitzvah, which is far greater in importance than the preparation, one should be guarded?
I would like to suggest an answer to the Taz’s question. When Sukkos comes and we have Baruch Hashem been successful in having all of our needs taken care of, one tends to let his guard down. Though we are surrounded by the sukkah, and are embracing the lulav, our minds are open to be occupied with ideas of lesser importance. Whereas during the time of preparation, we are worried and focused on how we are going to perform the mitzvah properly. It is very appropriate that at the first time a person’s guard is down, which is the first day of Sukkos, Chazal give us that warning that though you are been involved in the great mitzvah of yeshivas sukkah, don’t lose your focus!
If we start off on the right foot, without losing our focus on Sukkos, we will IY”H merit to have a year blessed with achieving our spiritual goals, and through that merit to the coming of Moshiach.