Mayor Kim Janey plans to sign a proposal this week that will move Boston’s preliminary election a week earlier to help accommodate work associated with mail-in voting.

City councillors passed the measure at their meeting on April 28 and in a letter to the body, former Mayor Martin Walsh said the change was necessary to help the city’s election department facilitate the preliminary and Nov. 2 general election in the fall.

“Due to a potential expansion of vote by mail to include the fall elections, the timeframe for certifying election results and the department’s requirement to hold a ballot position drawing, the original date would create a challenge for the printing and distribution of vote by mail ballots,” Walsh wrote in the letter.

The Legislature voted this session to extend voting by mail provisions through June 30 to help facilitate hundreds of municipal elections scheduled for the spring. Boston’s preliminary election is scheduled for Sept. 21 and when Janey signs the proposal into law, it will move to Sept. 14.

To be eligible to vote in the preliminary election, voters who are not currently registered must do so by Sept. 1 at 8 p.m. Boston voters in September will select two candidates for mayor who will face off in the Nov. 2 election.