I've tended to use diēs indictus, which I got from Livy:
Et apparebat nihil liberae consultationis concilio quod in diem posterum indictum erat Boeotis relictum esse. (33.1)
Hac quoque gente adiuncta Chalcidem regressus, praemissis inde litteris, ut Demetriadem conuenirent principes Aetolorum, cum quibus de summa rerum deliberaret, nauibus eo ad diem indictum concilio uenit. (36.6)
Glancing through the corpus at PHI, though, it looks like diēs cōnstitūtus, as offered by cnread, is more common.
