First try rebooting into Safe Mode by holding the Shift key down on your Mac when starting up.

This will force the system to run a drive check and clear some system caches.

Before You Begin Clearing Your Cache Community Q&A If you're running into problems with your Calendar app in OS X, such as long loading times or error messages, deleting your cache may help.

Before deleting your cache, try removing and re-adding your calendar accounts to see if this fixes the problem.

After upgrading to OS X 10.10.3, you may find that navigating the Finder can be arduously slow at times, where the contents of folders you open may take a while to display, and otherwise scrolling and navigating may be overall sluggish.If this occurs, then you can likely fix it by clearing some Finder-related cache information from your Mac.Caches are small databases of frequently accessed resources that a program can access far quicker than having to locate and parse this information directly from locations on disk, or which store specific URLs and other Web resources that a program may use for accessing an online account or other data.If you need to delete the cache, make sure to backup your calendars in case clearing the cache deletes them entirely.If you are a network administrator, web developer or a system administrator of Mac, you may face a necessary situation to flush DNS cache on Mac OS X.You get to clear either Unicast DNS or Multicast DNS, or both.If you are attempting to reset all DNS caches on the Mac, then you may have to consider clearing both as a proper measure.