I crossed the dateline, traveling eastward (from Australia to California), during Sefira. What should I do regarding Shavuos?
One who travelled east, observes Shavuos a day earlier than the people at his destination (i.e. on Shabbos and Sunday, the fifth and sixth of Sivan).
These Halachos are relevant for this situation:
- Shabbos: This day is Yom-Tov for the traveller but not for the local populace. The traveller should recite the Yom Tov Tefillos and Kiddush (adding the selections for Shabbos), but omit the three words “Zman Matan Toraseinu”.
- Sunday: This is the second day of Yom-Tov for the traveller but the first day for the local populace. The traveller observes Yom-Tov as usual, and he may include the three words “Zman Matan Toraseinu” in his Tefillos. At the end of this day, the traveller recites Havdallah in private.
- Monday: This day is Yom-Tov for the local populace but weekday for the traveller. The traveller should don Tefillin and daven the weekday Tefillos. Other than that, he should not perform any Melocho, even in private.
Whenever the traveller’s Tefillos are at variance with those of the local populace, he should daven discreetly and not call attention to the fact.
See here for a complete overview of the relevant Halachos: Dateline 5779.