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The overnight interbank rate EONIA= -- which typically rises as excess liquidity dwindles -- was expected to continue its downward trend during the current maintenance period.

Right, the maintenance periods are likely responsible for the heartbeat pattern. See ECB: Publication of the indicative calendars for the reserve maintenance periods in 2010 and 2011

The European Central Bank (ECB) is today publishing indicative calendars for the Eurosystem's reserve maintenance periods in 2010 and 2011. For the first time, these calendars are being published for the next two years, thereby following the practice adopted in setting the schedules for the meetings of the Governing Council of the ECB.

Keynesianism is intellectually hard, as evidenced by the inability of many trained economists to get it - Paul Krugman