Compiled Laws, Chapter 168. Michigan Election Law § 168.593

Sec. 593. A state convention of all political parties shall be held not less than 37 days before the first Monday of April in every odd numbered year. The state central committee of each political party shall cause to be forwarded by mail to the chairman of the county committee of such political party in each county a copy of the call for said state convention of such political party, showing the number of delegates to which each county shall be entitled in the state convention of such political party; and the said state central committee shall apportion such delegates to the several counties in proportion and according to the number of votes cast for the candidate of such party for secretary of state in each of said counties, respectively, at the last preceding November election. The particular day and the hour and place of meeting shall be designated by the state central committees of the various political parties in the call issued therefor by the respective state central committees of the several political parties. A certified copy of such call shall be immediately filed with the secretary of state.