According to a post by Wryxian on the EU forums Arena Season 5 will be ending April 14th and Arena Season 6 will begin one week later. Remember all arena points, teams and personal ratings will be reset with the new season.

You can read the original post after the jump.

Original post here:

We're currently aiming for April 14th as the end of Arena Season 5, though it may also be a week after that. When it ends, the process for determining eligibility for end of season rewards will probably take around one week. It's very important for players who feel that they may be eligible for the Deadly Gladiator’s Frostwyrm or Arena titles to refrain from transferring their character to another realm until after Arena Season 5 has ended.

Arena Season 6 will not commence until at least a week after this one ends. Between the end of Season 5 and the start of 6, only skirmish matches will be available to play.

When Arena Season 6 begins, arena points, teams and personal ratings will be reset, but the hidden matchmaking rating and honor points will stay in place.