A post from Blzzard on the offical forums details the testing schedule for Ulduar on the PTR.

On North American servers you will be able to test Hodir starting Thursday February 26th at 7PM EST (GMT -5) and the Iron Council starting Friday February 27th at 7PM EST (GMT -5).

On European servers you will be able to test Thorim starting Thursday February 26th at 7PM CET (GMT +1) and Freya starting Friday February 27th at 7PM CET (GMT +1).

All of this is subject to change at any time.

More info after the jump.

The original post:

Ok, we're going to attempt to have some playtest sessions for Ulduar starting tonight and through this weekend.

The following is the tentative schedule for this weekend:

On the North American test realms:
Hodir - Thursday night, February 26, starting at 7pm EST/4pm PST.
Iron Council - Friday night, February 27, starting at 7pm EST/4pm PST

On the European test realms
Thorim - Friday night, February 27, starting at 7pm CET.
Freya - Friday night, February 28, starting at 7pm CET.

This schedule could change at a moment's notice due to server, bug, or patch status. I'll try to keep everyone informed as much as possible. Ulduar and the bosses to be tested will remain also open a variable length of time. How long they stay open depends upon a host of factors including designer availability, bug status, etc.

We also hope to have the zone open on Sunday for each respective realm for a longer period of time. We have a couple special versions of Patchwerk setup. One of them is called Patchwerk (DPS Test) and is essentially a massive training dummy for your raids. The other is Patchwerk (Tank Test), and he does not have a Hateful Strike, but his melee damage increases over time.

We're very interested in seeing combat logs and parses for all raid testing on the PTR.

Quick Ulduar FAQ:

Q) Will there be testing available on Oceanic, Korean, or other time zones?
A) There will be some testing available. We do plan on allowing for extended testing on Flame Leviathan, and possibly other bosses. We want encounter designers to be present during testing, which makes testing during some time zones difficult.

Q) Where's all the trash?
A) We don't have everything turned on for these tests, and this sometimes includes trash creatures before each boss encounter.

Q) Where's the loot?
A) We don't have this attached to bosses yet, but we'll have loot hooked up soon.

Q) What about achievements?
A) These aren't implemented yet.

 

You can find the original post here.