They crossed the River Isen on Midsummer's Eve disguised as riders in black.
9 seconds sound clip from the The Lord of the Rings: The Fellowship of the Ring movie soundboard.
You can hear this line at 00:49:14 in the Blu-ray version of the movie.
Quote context
[...]
- They are not all accounted for, the lost Seeing-stones.
- We do not know who else may be watching.
- The hour is later than you think.
- Sauron's forces are already moving.
- The Nine have left Minas Morgul.
- They crossed the River Isen on Midsummer's Eve disguised as riders in black.
- They will find the Ring. And kill the one who carries it.
- Against the power of Mordor there can be no victory.
[...]