ANSWER TO RIDDLE

 

This special & unique day happens only once every few years. There are special Halachos that apply only for this particular day. This year תשפ"ג  in the month of  חדש סיון that special day will arrive. ? IF THE יום הטבוח FALLS ON A SUNDAY (This year*) THEN THE כהן גדול IS BANNED FROM DOING THE עבודה Individuals who couldn't be מקריב קרבנות on Yom Tov שבועות did so the following day. It is called "יום הטבוח" The day of the שחיטה.  It was considered more of a Yom Tov than אסרו חג Sukkos & Pesach.  Sukkos & Pesach individuals were Makriv their קרבנות on Chol Hamoed. On שבועות they did it the day after Yom Tov. The Mishna in Chagiga 17a says that if the יום הטבוח falls on a Sunday then the כהן גדול doesn't wear his clothes, fasting & Hesped are allowed to exclude from the צדוקים who claim  that שבועות is always on מחרת השבת. on Sunday. Tosfos explains that the כהן גדול usually used to do the עבודה  לכבוד יום טוב When  יום הטבוח fell on a Sunday the כהן גדול would not dress in the בגדי כהונה and did not do the עבודה so people shouldn't confuse it to be a Yom Tov.     *assuming 2nd day is Shavuos