The beginning of Parshas Shmini, usually read at the end of Nisan, refers back to its beginning: It was on the eighth day of the inauguration, Rosh Chodesh Nissan, the day the Mishkan was erected (Rashi Vayikra 9:1).
At the midpoint of Nisan, on Seder night, we begin with disgrace and conclude with praise (mishna Pesachim 116a). According to Shmuel, we begin with "We were slaves to Pharoh in Mitzrayim" and end "Hashem took us out from there". According to Rav, we begin with "At the beginning our ancestors were idol-worshipers" and end "and now Hashem has drawn us close to His service".
At the Seder, we follow both Shmuel and Rav. Shmuel focuses on the micro level, on the slavery in Egypt and the Exodus therefrom. However Rav looks at the "big picture". Our spiritual enslavement began when our ancestors worshiped idols. It ended when we served Hashem, our ultimate spiritual redemption.
The book of Shemos, called the book of exile and redemption, ends with the completion of the Mishkan. Even after we left Mitzrayim we were still in exile until we returned to the level of the patriarchs when Hashem's presence was in our midst in the Mishkan (Ramban, introduction to Shemos). This took place on Rosh Chodesh Nisan (Rashi Vayirka 9:23).
Based on this, my son R' Moshe explained two famous passages in the Haggadah.
The final portion of our description of the Exodus (before Rabban Gamliel omer, the subject of the next mishna in Pesachim 116a) is "Dayenu". Its last stanza refers to the building of the Beis Hamikdash, the return, in the words of the Ramban, not only to the level of our patriarchs, but to their place as well. This represents the completion of Hashem's drawing us close to His service.
The paragraph preceding the disgraceful beginning of idol worship suggests that we tell the story on Rosh Chodesh Nisan. On a simple level, this suggestion is based on the references in Parshas Bo (Shemos 12:2; 13:4) to the month of mitzvos and exodus. However, one can interpret this suggestion as referring to the glorious end of our spiritual redemption, which took place on Rosh Chodesh Nisan in the year after the Exodus.
When reading about the day when we returned to the level of the patriarchs, we should all aspire to that goal. Each and every Jew is obligated to say "When will my deeds reach the deeds of my ancestors, Avraham, Yitzchak and Yaakov"? (Tana D'bei Eliyahu 25:2). Aspiring to that goal, even if it is beyond our reach, is the very essence of the lesson we must derive from the Haggadah. We pray for the day when the goal will be realized with the rebuilding of the Beis Hamikdash.