Tum, paucos dies commor- atus, ut ad omnes casus subsidia compararet, navem deduxit et tempestatem ad navigandum idoneam nactus, magno cum plausu omnium solvit. Look at the demonstrative ea that agrees with it.
Dies is always masculine in the plural and frequently so in the singular.
But mostly it's feminine in the singular. Compare this construction to the one you saw in Hercules 1.2 . Tough one here. You're in indirect statement after arbitratus est . Did you remember what to do with ad subeunda pericula .