Going by an objective standard, I notice that Union is at 2 yellow dots around 7-10PM GMT(London) time.
Otherwise I would say that from 1AM-6AM is likely not-so busy.
In between these times, especially with summer now here, it should be reasonable. Make sure you join the channels!
Fury
Thank you for the info - looks like Union will have to be my Saturday afternoon hotspot. 
Laile - @Kaie
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Yes, the glass is half full, there is always another perspective and no they are not following you.
Can anyone tell me when Union is busiest? I have tried and like playing on the server but am having trouble figuring the peak play times. Please note the times with time zones so I can convert to my local.
Thank you ~
Laile - @Kaie
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Yes, the glass is half full, there is always another perspective and no they are not following you.