Reference for Menachot 80:1
חמץ בפסח ויוה"כ דכרת סמכינן אדיסקי הכא דעשה בעלמא לא כל שכן
in respect of leaven on the Passover Festival or in respect of the Day of Atonement which involve the penalty of kareth we rely upon public notices,<span class="x" onmousemove="('comment',' The decisions of the Sanhedrin concerning intercalation of the year whereby the year is deemed to be a leap year and thus postponing the Passover Festival for a month, or intercalation of the month whereby another day is added to the month and thus postponing the Day of Atonement (or any Festival that comes in the subsequent month) by one day, were announced to the public by means of notices and letters.');"><sup>1</sup></span> how much more so may we rely upon them here where only the transgression of a positive precept can be involved!<span class="x" onmousemove="('comment',' For the use of the test thread (or the thread dyed from the quantity taken for testing) is but an infringement of a positive precept, for Holy Writ declares, That they make them fringes (Num. XV, 38) , that is to say, the threads must be prepared specifically for the zizith.');"><sup>2</sup></span>