In southern Austria I have not come across this wording yet. Might be something specific to Germany.
An article in the Main Post titled "Ein Strohkranz als Zeichen der Schande" (A straw garland as sign of disgrace) from Schernau, Kitzingen (still Bavaria but not so far away from Thuringia) suggests that both @ColeValleyGirl and @Til Hund seem to be right.
It seems that the expression "in der Bethstund" means that the bride either was pregnant, there already was a child or either bride or groom had attracted attention wrt fornication in the past. While there were several hours for prayer (I would have spontaneously attributed the "Bethstund" to 6 p.m.) in that case it seems to be the one at midday.
In detail the article states:
[...] Zum Beispiel in Schernau anno 1590. Da brachte ein Schernauer Paar ein vierteljähriges Kind mit zur Trauung. Die Zeremonie verlief dann "ohne Kranz, Haarband und Saitenspiel". [...] In der Praxis bedeutete dies, dass das Paar ohne "Haarband und Ehrenkranz", auch "Jungfernkranz" genannt, woraus sich allmählich der Schleier entwickelte, zur Trauung schritt. Als Zeichen der "Schande" trug ein Büttel ein Strohkränzchen voraus. Gesang und Orgelspiel waren untersagt wie auch fröhliches Hochzeitstreiben. Der Gottesdienst fand in aller Schlichtheit und Heimlichkeit entweder nach der Frühmesse oder der mittäglichen Betstunde statt. In vielen Fällen wurden [sic] die Zeremonie hinter einem Vorhang vollzogen.
[...] For example, in Schernau in 1590, a Schernau couple brought a quarter-year-old child to the wedding ceremony. The ceremony then was conducted "without garland, hair ribbon and strings". [...] In practice, this meant that the couple went to the wedding without "hair ribbon and garland of honor", also called "maiden garland", from which the veil gradually developed. As a sign of "disgrace", a bailiff carried a garland of straw in front of the couple. Singing and organ music were forbidden, as was merry wedding bustle. The service took place in all simplicity and secrecy either after the early mass or the midday prayer hour. In many cases the ceremony was performed behind a curtain.