Section 17. At least twelve weeks prior to the time when the services of grand or trial jurors are required, the clerk of each court requiring such jurors shall send or deliver a letter of venire to the office of jury commissioner. The letter of venire shall state the number of jurors required for grand jury service or for trial jury service, the court, and the judicial district. In the case of grand juror service, the letter shall state the beginning date and the estimated length of the term. In the case of a special grand jury, the letter may require the jury commissioner to summon jurors to appear for service forthwith or within a lesser period than would otherwise be required under this section. In the case of trial juror service, the letter shall state the period or periods of juror service to which the letter applies and the number of jurors required for each day during each period. The letter shall contain any further information which the jury commissioner deems appropriate.