Updates are typically released by Microsoft at 10am PDT (5pm UTC).
Configuring WSUS servers to synchronize relative to that time can be helpful in expediting availability of these security updates.
Windows XP End Of Support - in case you haven't already heard... TODAY is the last day that updates for Windows XP will be released to the public. No additional updates will be available for Windows XP (unless you've purchased a Microsoft Premier Support Contract that covers additional security updates for Windows XP). You should also be prepared to hear about new vulnerabilities in Windows XP and Internet Explorer v6, v7, and v8, that will not be patched... possibly as soon as Wednesday, but likely for a very long period afterward. Ensure that you have current AV/AM software installed on any remaining Windows XP systems, and I would also suggest switching those users from IE8 to the current version of Firefox or Chrome if its is necessary to use the Internet from a Windows XP system. Best case: Windows XP should not be used to access the Internet or read email.
Windows 8.1 / Windows Server 2012 R2 UPDATE -- Also on this Patch Tuesday (April 8, 2014), Microsoft is releasing the UPDATE for Windows 8.1 and Windows Server 2012 R2 systems. This UPDATE is MANDATORY -- in that all subsequent updates for Win8.1/WS2012R2 systems will be baselined from this update. That is to say, you WILL NOT be able to install the May 2014 updates to a Win8.1/WS2012R2 system unless you first have installed this April update. In addition, I've read reports that this update can take a LOOONG TIME to install... so plan accordingly. I would also NOT install this update in combination with anything else, and I would install it *AFTER* you've installed the regular April Security Updates for Win8.1/WS2012R2/IE11. Some have even suggested doing a FULL SYSTEM BACKUP prior to launching the installation of this update.