Wherefore all the men of Israel assembled themselves unto the king in the feast which was in the seventh month.
This agrees with what we had Kg1 8:2, etc., where an account was given of the solemn introduction of the ark into the new-erected temple. 1. There needed no great solemnity for the bringing in of the dedicated things, Ch2 5:1.
Commenting on 2 Chronicles 5:1-10
Wherefore all the men of Israel assembled themselves unto the king in the feast which [was] in the seventh month. (b) When the things were dedicated and brought into the temple. (c) Called in Hebrew Ethanim, containing part of September and part of October, , which moves the Jews called the first month, because they say that the world was created in that month, and...
The feast - "That is, the feast of tabernacles, which was held in the seventh month." - Targum. See Kg1 8:2.