We meet in a rented hall so we don't have a bell or buzzer to use to alert everyone when services begin. During our business meeting last evening one brother (whose wife teaches the children's class) made a request. He stated that our normal procedure during our mid-week services has been that we make announcements, have a couple of songs then the lady teaching the children goes to another room for a class.
Sometimes she does not have time to gather up her class materials and return for the invitation that is extended; therefore, he suggested that we offer it at the beginning of the class. I have never seen this practiced and suggested that one of us go and signal her in order that she will have time to prepare by gathering up her class materials and return to the auditorium.
I know of nothing in the Scriptures indicating that an invitation must be offered each time we gather, let alone when that invitation is given. Generally, congregations wish to take advantage of the time that they have to encourage one another toward faithful service. An invitation naturally follows a lesson (short or long) because the lesson should stimulate the audience's mind toward things which need correcting or improving. Most people feel that it makes a good ending to a gathering, but I've seen it done at other times. Here in La Vista, on Sunday evenings the lesson and invitation is given in the middle of the service instead of at the end. It basically comes down to what is expedient for the local group. I know of one congregation where the preacher rarely offers an invitation at the service. He doesn't want people to think there is a set time to obey the gospel.