Thursday [07/12/2017] at 5:00am EST [11:00 CET] we'll be doing a full maintenance on all our services, which means that HMM will be unplayable for the entirety of the maintenance. We expect everything will be back online by 11:00am EST [17:00 CET].
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Scheduled Maintenance - Thursday [07/12/2017] at 5am EST [11:00 CET]
Source: Steam Community (opens in a new tab)