旅行できる日

the day when we can travel

→ 旅行
phrase Rare #39,230
the day when we can travel Refers to a future time when travel to a particular destination becomes possible. The subject of 旅行できる is often understood from context.
JA: いつか火星へ旅行できる日が来るでしょう。
EN: Someday, the day when we can travel to Mars will come.

月へ旅行できる日まもなく来るだろう

The day when we can travel to the moon will come soon.

私たちが月に旅行できる日来るだろう

The day will come when we can travel to the moon.

宇宙旅行できる日やって来た

The day has come when we can travel through space.

火星旅行できる日遠くあるまい

The day when we can travel to Mars may not be far off.