In Ezekiel 45:18-20, God commands a sin offering on the first of the month, and another on the seventh of the month for
everyone who goes astray or is naive [other translations read simple-minded]; so you shall make atonement for the house. (NASB)
Is this second sin offering exclusively for people who sin unintentionally? If so, what would be the purpose of having this offering separate from the sin offering on the first of the month?
The reference in Ezekiel seems to be to prepare the sanctuary at the beginning of the year and prior to the Passover season. The sacrifice of a bull for atonement is for the community, not an individual. I suppose a bull is for a Priest that sins as well - Lev. 4:3.
There are a couple of different places that mention unintentional sin of an individual:
Leviticus 4:27-31
And if one of the common people sins unintentionally and does what is prohibited by any of the LORD’s commandments, he incurs guilt. When he becomes aware of the sin he has committed, he must bring an unblemished female goat as his offering for that sin. He is to lay his hand on the head of the sin offering and slaughter it at the place of the burnt offering. Then the priest is to take some of its blood with his finger, put it on the horns of the altar of burnt offering, and pour out the rest of the blood at the base of the altar. 31Then he is to remove all the fat, just as it is removed from the peace offering, and the priest is to burn it on the altar as a pleasing aroma to the LORD. In this way the priest will make atonement for him, and he will be forgiven.
The same is essentially repeated in Numbers 15:27-29
Also, if one person sins unintentionally, he is to present a year-old female goat as a sin offering. And the priest shall make atonement before the LORD on behalf of the person who erred by sinning unintentionally; and when atonement has been made for him, he will be forgiven. You shall have the same law for the one who acts in error, whether he is a native-born Israelite or a foreigner residing among you.
There is also a daily sacrifice at 9 AM and 3 PM called the Tamid - this daily sacrifice could also be considered for minor transgressions, missteps, and unintentional sin of an individual.
Exodus 29:38-41
“This is what you are to offer on the altar regularly each day: two lambs a year old. Offer one in the morning and the other at twilight. With the first lamb offer a tenth of an ephah c of the finest flour mixed with a quarter of a hind of oil from pressed olives, and a quarter of a hin of wine as a drink offering. Sacrifice the other lamb at twilight with the same grain offering and its drink offering as in the morning—a pleasing aroma, a food offering presented to the Lord.
