The Gemara says that Moshiach can come Erev Shabbos because it won't be a Tircha, being that everyone will be our servants.
On the other hand, if someone says that he should become a Nazir the day Moshiach comes, he is a Nazir Misafek even on Shabbos -- if he said this before Shabbos. Rashi explains that this is because Moshiach might have appeared to the Beis Din Hagadol in Yerushalayim and this person didn't hear of it.
My question is, if it is possible that he didn't hear of it, how would the Goyim hear of it, enough to be convinced to help us.
Haleivi
The Gemara says that Eliyahu might appear before the Beis Din ha'Gadol, not Moshi'ach. Apparently, when Moshi'ach arrives everyone will know about it immediately.
Be well,
Mordecai Kornfeld
Kollel Iyun Hadaf
That is what the Gemara says, regarding why you have to be Choshesh that Moshiach will still come today -- although you didn't hear about Eliyahu Hanavi coming yesterday.
Rashi however, addresses the case where he said, "Hareini Nazir Beyom Sheben David Ba" on Friday. In that case, why would he be a Nazir on Shabbos? Moshiach can't come on Shabbos -- because Eliyahu Hanavi can't come on Friday -- and Moshiach didn't come before Shabbos because I and my Goyishe neighbors didn't hear about it.
To this Rashi says that we say, perhaps Moshiach did come on Friday -- to the Beis Din Hagadol -- and you just didn't hear of it.
(This question didn't bother Tosafos because he learns that the Neder is to be a Nazir on a day that Moshiach can come. Therefore, my question is not on Tosafos, either.)
Haleivi
Rabbi Yaakov Emden (printed in back of most editions of the Talmud) writes that when Eliyahu Ha'Navi comes to the Beis Din Hagadol, everyone in Yerushalayim will know but people outside of Yerushalayim might not know.
When the Moshiach arrives on the following day, the entire world will know, so if the Mashiach comes on Friday everyone will want to help Klal Yisrael prepare for Shabbos.
Dovid Bloom
Kollel Iyun Hadaf
Rashi, Dibur Hama'chil Kiven D'chal Alei Neziros, explains the Gemara that on Shabbos we say that Moshiach might have come Yesterday to the Beis Din Hagadol -- although you are unaware of it.
Besides, quoting Reb Yaakov Emdin here is irrelivant. He simply says that when the Beis Din Hagadol knows the whole Yerushalayim will know. This says nothing about the question.
1. Thank you for pointing out that Rashi writes that the Mashi'ach himself may have come to the Beis Din ha'Gadol but still not everyone will know about it. We will have to retract from our original answer that when Mashi'ach arrives, everyone will know about it immediately.
2. Instead, the distinction that we must make here is that when the Mashi'ach comes, everyone in the place he comes to will know about it. As soon as everyone knows, then the entire local population will want to help Yisrael. However, in other places it might take them longer to find out about it.
3. I think that this is where Rav Yakov Emden helps us, because he implies that when the Mashi'ach comes to the Beis Din ha'Gadol everyone in Yerushalayim will know about it, but Jews outside Yerusahalayim may still not know.
Kol Tuv,
Dovid Bloom