Why do we celebrate rosh Hashanah two days? Why do the reform Jews celebrate one day?

Shalom, The Torah tells us to celebrate one day of Rosh Hashana. We celebrate Rosh Hashana for two days primarily because the Jewish calendar is based on the sighting of the new moon. Each month, as well as the New Year, was established by witnesses who saw the new moon, testifying before the Sanhedrin (the Jewish Supreme Court) and they then declared the new month as begun. All the other festivals do not fall on the first of the month, and so there is time for the declaration to be made and the date of the festival to be announced. Rosh Hashana, though, falls on the first of the month, and as such, it was not certain which day the witnesses would come to the Sanhedrin, as the moon's cycle is not exactly 30 days, but slightly less, causing some months to have 29 days and others 30. Because of this doubt, it turned out that sometimes people celebrated one day of the festival, and then it turned out that the witnesses did not come and they had to keep a second day. It was later decided to always keep two days of Rosh Hashana to cover the eventuality that the witnesses would come on either of the two possible days and not have people unsure of the correct day. (Though the Talmud describes various historical stages that we went through before reaching the present way of keeping two days, this is the general idea). Even though we today base our calendar on calculations, and so know the correct date in advance, the original decree was not (and cannot at present) be canceled. Despite the technical reasons for this, involving the structure of the entire make-up of Jewish law, there are also deep spiritual and social reasons to keep the law as it is - at least until we re-establish a Sanhedrin. But even then it appears likely that the law will remain as it is now, as we will return to establishing the new month based once again on witnesses. I am unable to speak for the reform movement. The celebration of two days of Rosh Hashana dates back to early rulings of the Prophets, and is unquestioned Jewish practice as recorded in the Talmud and rabbinic literature in every Jewish community throughout the world