[1] The International dateline refers to an imaginary line which cuts through the globe and separates between where on the globe the day begins and where the day ends. For secular legal terms the dateline runs through the Pacific between Alaska/Hawaii and Russia/Australia, and hence Russia/Australia is the first to start its day while Alaska and Hawai are the last. Thus there is an almost 24 hour difference between those countries, even though they are not far in distance. There is a dispute amongst later Poskim regarding the Halachic positioning of the International dateline. Some Poskim [Chazon Ish] rule that Japan is west of the Dateline and thus is the last country to begin its day. Thus while according to the legal dateline Japan is the first to begin the new weekday, according to Halacha it is the last. Thus while according to the legal dateline Japan is beginning Saturday so to say, according to the Halachic dateline [according to the Chazon Ish] it is really beginning Friday, and thus Shabbos is really on its Sunday. Many other Poskim however argue with this opinion and rule that Japan is on the east of the dateline, as states the secular legal dateline and its Saturday is the correct day. Below we will discuss final rulings on this matter.

[2] Rebbe in Shaar Halacha Uminhag  2/220; Sheivet Haleivi 3/28 and 6/34 explains the importance of the fact that the custom always was to keep Shabbos on Saturday in Japan and thus one must keep Shabbos on that day for all matters, and G-d forbid to desecrate any part of it. However the Sheivit Haleivi himself concludes that also on Sunday no Melacha should be done as will be brought next.

[3] Sheivet Haleivi 6/34; The Rav asking the question to the Sheivet Haleivi 3/28.

[4] In Sheivet Haleivi 3/28 the questioner writes that he would tell askers to put on Tefillin without a blessing on Shabbos. However the Sheivet Haleivi negates this.

[5] Yisrael Vehazmanim 78

[6] Such as regarding prayer, Tefillin, Rabbinical Melacha.