Yosef Karo

“ Paragraph 2 - At twilight, the eruv must be in a place where it is possible to take it. Therefore, if a stone fell on it and it cannot be picked up without a hoe or pick-axe, it is forbidden [to rely on the eruv] . Rem"a: The eruv only needs to exist at twilight (Tur) . One can eat it once it is definitely dark (Hagahot Oshr"i and the Mordechai in Chapter Bechol M'arvin) . One should eat it on Shabbat morning (Minhagim) . This is only true in a place where they make an eruv every Friday, but it is better to make an eruv once for the whole year, as was said above in chapter 368 (Beit Yosef) . ”