As I understand it, the origin for having a 2nd day of Rosh Hodesh & Yom Tov is that messengers were only able to travel a certain distance from where the Sanhedrin was to convey the declaration of the actual date of Rosh Hodesh. Now, that we have a fixed calendar, we keep the old custom.
My second understanding is that during Mashiach, everyone will be living in Israel, and we will revert to witnesses and Sanhedrin declaring Rosh Hodesh. However, I assume also, that not everyone will be able to live close enough to the Sanhedrin to get the message of Rosh Hodesh imediately, IF we assumed that the same system of messaging will be used as was before.
Question - Let's say, the future Sanhedrin were able to use current technology such as the web and SMS. Now, anyone with access could find out almost instantaneously when Rosh Hodesh was declared, so no one living anywhere would have a doubt when the date was. Would the use of technology eliminate the need for 2 days Rosh Hodesh & Yom Tov, or would we still be following the ancient custom of our ancestors?