Other posts on Bamidbar:
What about me?
Silence is golden
וַיְדַבֵּר ה’ אֶל מֹשֶׁה בְּמִדְבַּר סִינַי בְּאֹהֶל מוֹעֵד בְּאֶחָד לַחֹדֶשׁ הַשֵּׁנִי בַּשָּׁנָה הַשֵּׁנִית לְצֵאתָם מֵאֶרֶץ מִצְרַיִם לֵאמֹר- Hashem spoke to Moshe in the Sinai Desert, in the Tent of Meeting on the first day of the second month, in the second year after the exodus from the land of Egypt, saying. (1:1)
באחד בניסן הוקם המשכן, ובאחד באייר מנאם- Rashi explains, When He came to cause His Divine Presence to rest among them, He counted them. On the first of Nissan, the Mishkan was erected, and on the first of Iyar, He counted them.
A question arises. Why weren’t they counted already by the first of Nissan? Rashi mentions it had something to do with the shechina coming down to Bnei Yisroel and that had already occurred on the first of Nissan.
דַּבֵּר אֶל בְּנֵי יִשְׂרָאֵל וְיִקְחוּ לִי תְּרוּמָה מֵאֵת כָּל אִישׁ אֲשֶׁר יִדְּבֶנּוּ לִבּוֹ תִּקְחוּ אֶת תְּרוּמָתִי- Speak to Bnei Yisroel, and have them take for Me an offering; from every person whose heart inspires him to generosity, you shall take My offering. (Exodus 25:2)
אמרו רבותינו שלש תרומות אמורות כאן, אחת תרומת בקע לגלגלת, שנעשו מהם הא-דנים ואחת תרומת המשכן נדבת כל אחד ואחד- Rashi mentions the three times Bnei Yisroel were counted during their first year after leaving Egypt. One of them is when each member of klal Yisroel gave half a shekel for the sockets of the mishkan. This was, of course, before the first of Nissan, before the mishkan was set up.