From BiblePortal Wikipedia

Lo! [1]

1: ἴδε (Strong'S #2396 — particle — ide — id'-eh )

an aorist or point tense, marking a definite point of time, of the imperative mood of eidon, "to see" (taken as part of horao, "to see"), is used as an interjection, addressed either to one or many persons, e.g.,  Matthew 25:20,22,25;  John 1:29,36,47;  Galatians 5:2 , the only occurrence outside Matthew, Mark and John. See Behold , See.

2: ἰδού (Strong'S #2400 — particle — idou — id-oo' )

a similar tense of No. 1, but in the Middle Voice, e.g.,  Matthew 1:20,23; very frequent in the Synoptists and Acts and the Apocalypse.