Verses 12 and 13 are a single long sentence in the Hebrew text, which Revised Standard Version reflects. However, Good News Translation makes a separate sentence of verse 13. A number of modern versions do something similar in order to break up the long sentence in the original text (so Contemporary English Version, New Century Version, Parole de Vie).
As the duty of each day required, offering according to the commandment of Moses: Solomon followed the Law of Moses when he offered sacrifices on the Temple altar. He did what Moses required for each holy day. As the duty of each day required is repeated in the next verse. The beginning of this verse may be rendered “He offered sacrifices in a manner that followed the rules in the Law of Moses for each holy day.”
The sabbaths: See the comments on 1 Chr 9.32. Instructions for offering sacrifices on the Sabbath are found in Num 28.9-10.
The new moons: See the comments on 1 Chr 23.31. Instructions for offering sacrifices on the New Moon Festival are given in Num 28.11-15.
The three annual feasts: See Exo 23.14-17; 34.22-23; Deut 16.16. Using more common language, Good News Translation says “the three annual festivals.” These three festivals are specifically named in the text that follows.
The feast of unleavened bread was held in the Hebrew month called Nisan, from the fifteenth day until the twenty-second day, which is roughly equivalent to early April in the modern Western calendar. This festival took place immediately after Passover, also in commemoration of the deliverance of the Israelites from their slavery in Egypt. Its name came from the practice of not using leaven (yeast) in making bread during that week. Instructions for offering sacrifices during this festival are given in Lev 23.6-14 and Num 28.16-25.
The feast of weeks was celebrated fifty days after Passover, at the end of the wheat harvest. It is mentioned by this name only in Exo 34.22; Num 28.26; and Deut 16.10, 16. It is referred to as “the feast of harvest” in Exo 23.16. Good News Translation translates it here and throughout the Old Testament as “the Harvest Festival.” The Jewish name for this festival is Shavuoth (the Feast of Weeks). Later it became known as Pentecost (2 Maccabees 12.32), and this is the name that was eventually adopted by Christians (Acts 2.1). Instructions for offering sacrifices during this festival are given in Lev 23.15-22 and Num 28.26-31.
The feast of tabernacles: See the comments on 2 Chr 5.3.
Quoted with permission from Omanson, Roger L. and Ellington, John E. A Handbook on 1-2 Chronicles, Volume 1. (UBS Helps for Translators). Miami: UBS, 2014. For this and other handbooks for translators see here .