Why do we (in Orthodox congregations) announce the ‘molad’ the Shabbat before a new month? Since our calendar is fixed, what difference does it make?

Take this past week as an example. In our synagogue we announced that the molad was some time on Sunday morning (forgive me for not remembering the exact time). In that case, Rosh Chodesh should be on Monday. But it’s not. The new month of Sivan begins on Tuesday. So why bother announcing something that has no bearing on anything and which at best is just a vestige of a prior practice and at worst may serve to confuse people as to which day to commemorate the new month?

  • "In that case, Rosh Chodesh should be on Monday." Why?
    – Double AA
    Jun 6 '16 at 16:19
  • If the molad was Sunday morning then eidim would have come to the beis din Sunday night or Monday and that day would have been declared Rosh Chodesh, no?
    – TheRiver
    Jun 6 '16 at 16:34
  • It seems somewhat unlikely that any Eidim would have seen the new moon a mere 7 hours after conjunction. You happen to be right that Sivan is not tied to Molad Sivan, but the Molad is used actually to calculate the date of Rosh HaShana and thereby the rest of that year's months which are fixed relative to it.
    – Double AA
    Jun 6 '16 at 16:49
  • Hello TheRiver, welcome to Mi Yodeya, and thanks very much for the interesting question! I hope you find more Q&A of interest and stay learning with us!
    – mbloch
    Jun 6 '16 at 18:11
  • 1
    Note also that even if aidim had seen the moon and made it to bais din, the minimum is 29 days. Thus, Rosh Chodesh could not have been on Monday (29 Iyar). Tuesday had to have been the earliest possible day of Rosh Chodesh, no matter when the molad occurred. Jun 6 '16 at 19:58

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