Every church I've attended that claims to be Reformed has taught that the Sunday church meeting (aka worship service) is the central and most important event of the week for Christians. If somebody had to decide to attend only one church activity during the week, the worship service should be it. It contains the central focus of worship - the preaching of, and thus the listening to - the sermon, and singing and praying to the Lord.
Some people have neglected to attend on Sunday, but make other meetings during the week their main focus. Inquiring leaders might get the reply, "But the midweek group is my church," and use the reply to warn against doing this.
But I'm wondering if those people aren't on to something. Maybe they sense that those other weekly gatherings have more fellowship, more one-anothers, more stimulating one another to love and good deeds, more encouraging of one another (Heb 10:23-25) that is supposed to go along with not neglecting to assemble together. And as Eric Carpenter writes at A Pilgrim's Progress, a former church of his had a built-in fellowship time that was very much enjoyed. Arthur Sido, in the comments, notes that a former church his family attended had a difficult time getting people to stop fellowshipping during a built-in time so they could attend the formal teaching. If more of the one-anothers that are listed in the New Testament occur in settings other than the Sunday meeting, why not allow somebody who realized this the benefit of the doubt in meeting during those other times?