Act 1 took place during the feast of Lupercal, February 15. Act 2 took place on March 14. Act 3 took place on March 15, the ides of March. Acts 4 and 5 took place a few months later.
Allusions in three contemporaneous works support a date of 1599 for julius Caesar.
1. Ben Jonson's play Every Man Out of His Humour (acted 1599,published 1600) paraphrases Shakespeare's line "O judgment, thou art fled to brutish beasts" (Julius Caesar, III,ii,104) as "reason long since is fled to animals" in III,i. Jonson's play also includes "Et tu, Brute" in V,iv.
2. The anonymous play The Wisdom of Dr. Dodipoll (published in 1600) gives its own paraphrase, "Then reason's fled to animals, I see."
3. A passage in John Weever's Mirror of Martyrs, published in 1601, makes clear reference to the speeches of Brutus and Mark Antony in Julius Caesar. John Weever stated that he'd written his poem two years earlier, which (presumably) fixes the date as 1599.