The mishkan was inaugurated on the first day of the Hebrew month of Nissan of the year 2449 from creation (1312 BCE). There is a mostly chassidishe minhag to recall this event by reading the parshiyos of Nesiim from the Torah in parshas Naso: this innovation was instituted by the Shelah HaKodesh (p.44) and perpetuated by mystics following him, such as the author of the Kav HaYashar and it was later adopted by chassidim. This custom has been a subject of discussion, being that we do not otherwise commerate such one-time historical event. We do not, for example, recollect the dedication of either of the Temples in Jersusalem, the day the Jews entered Israel or the fact that the mishkan was completed on the 25th of Kislev, as an antecedent to the holiday of Channukah! One explanatioin is that the portion of Nesiim is different becasue it expresses achdus/unity in longing for the redemption.
Moreover, it has been discussed that each of these twelve days of dedication, Nissan 1-12 are the first twelve days of the year and that they correspond and encapsulate within them each of the 12 months of the year . See here ( on p.458/9), and here . This is connected to the tikkun of souls which occurs before Tishrei as stated in Zohar Balak III, 196b and discussed here. The last 12 days of Elul also correspond, each day to a month, and make a tikkun on the corresponding month of the year. Elul and Nissan are connected becasue there are two opinion regarding the creation of the world in Masechta Rosh Hashanah as discussed in Sefer haSichos 5703 (Hebrew edition p.179).
The real deal is that these holidays have to do with the the concept of history being relevant today and not just being about the past:
“The Rebbe often quotes the AriZal’s interpretation of the verse in the Megillah : Hayamim ha-eleh nizkarim venaasim — literally, “These days are remembered and done.” However, the AriZal explains that every year when the holiday comes around (not only Purim, but any holiday or festival), the same spiritual energy that was first revealed on that date, is revealed again, every year. Moreover, we have to remember what happened then, and do something about it so that it will happen now as well. These days are here for us to do. We’re not just going to sit and reminisce about the old days, drink and be merry, because of a date on our calendar. For Yidden , all of our holy days and celebrations are an opportunity to do something, to affect us and to change us and to make us and the world better.” See here.