YHWH Speaks from the Tent of Meeting at Sinai
Numbers begins with Israel still at Sinai, but something crucial has changed since the beginning of Exodus: the Tabernacle is now standing in the middle of the camp, and YHWH speaks to Moses from the Tent of Meeting. The date places this scene on the first day of the second month in the second year after the exodus—exactly one month after Exodus says the Tabernacle was erected.
Where this fits in the story: Leviticus 27:34 closes Leviticus at Sinai, and Numbers 1:1 immediately resumes the Sinai setting. Exodus 40:17–21 supplies the dated bridge: the sanctuary was erected one month earlier.