"וספרתם לכם ממחרת השבת ... תספרו חמשים יום ... וקראתם בעצם היום הזה מקרא קדש"
“You shall count for yourselves from the morrow of the Shabbat ... you shall count fifty days ... you shall convoke this very day a holy convocation” (Vayikra 23:15-23)

QUESTION: What halachahic implications are there for each person’s Shavuot celebration due to his personal Sefirah — Omer counting?

ANSWER: The Torah usually gives a precise date as to when a festival shall be celebrated; e.g., the Torah designated Nissan 15 as Pesach and Tishrei 10 as Yom Kippur. The only exception to this is Shavuot. No specific date was assigned; rather, the Torah says that the date the Festival is celebrated is the 50th day of Omer counting which commenced on 16 Nissan.

Therefore, the Sages (Rosh Hashanah 6b) say that Shavuot can be on the fifth, sixth or seventh of Sivan, depending on how many days were in the months of Nissan and Iyar. If they were both full 30-day months, Shavuot will be on the fifth of Sivan. If both were only 29 days, Shavuot will be 7 Sivan, and if one month was 30 days and the other 29, Shavuot will be on 6 Sivan.

Counting the days of Sefirah is a mitzvah incumbent on each individual and is not comparable to the Jubilee Year (Yovel) which the Beit Din counts on behalf of Klal Yisrael. Thus, the Torah declared “usefartem lachem” — “you shall count for yourselves” — and upon concluding the counting of 49 days, the time arrives for you to celebrate the Yom Tov of Shavuot.

Normally, all the individuals in a locale start counting together the first day and continue doing so until they all finish counting 49 days. However, an interesting case arrises if one crossed the International Date Line in the middle of the Omer count.

While traveling from America to Australia, one loses a day, as it were. Hence, if one left America when counting the 14th day of Sefirah he arrives in Australia approximately a day later and at night they are counting the 16th Sefirah. Since technically this person only had 48 days of counting how should he conduct himself in regard to observing the holiday of Shavuot, whose celebration is dependant solely on the counting of the Omer?

The Lubavitcher Rebbe has a novel but halachically sound approach to this. In his opinion the individual continues counting his Sefirah until he concludes his 49th day of counting — then he starts to observe the Shavuot festival. Thus, according to the Australian calendar he will be celebrating Shavuot on the seventh and eighth of Sivan.

Likewise, if one travels from Australia to America in the middle of Sefirah, upon crossing the International Date Line, he will be gaining a day over the people in America, and his 49 days of counting will conclude a day earlier. Thus, he should celebrate Shavuot on the fifth and sixth of Sivan. (In such a case, he should not say in his tefillot or Kiddush of the first day, “zeman matan Torateinu,” since the Torah was not given on the fifth of Sivan.)

(לקוטי שיחות ח"ג ע' 998)