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based on Mondays and Thursdays (is it still this with daily extended range forecasts?)

In fact it is essentially the same re-forecast runs that are utilised to build the M-Climate and the ER-M-Climate; the key difference is that those runs are grouped together in different ways.

ER-M-climate is updated twice a week, every Monday and Thursday, and based on 00UTC runs only (there are no 12UTC re-forecast sets). 

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Different reference periods for M-Climate and ER-M-Climate

ECMWF uses different length reference periods to define essentially the same re-forecast runs that are used to build the M-Climate and the ER-M-Climate.   This is because they are The key difference is that those runs are grouped and used in different ways. 

• For shorter ranges, the priority is the best possible capture of the climatological distribution of the tails.  It has been shown that using 1980 realisations (spanning 4 weeks) achieves this much better than using 660 (spanning 1 week). 
• For longer ranges, the priority is the correct representation of seasonal cycles.  This can be better achieved by spanning 1 week rather than 4.  The extended range uses week-long averaging and the tails should not be so prone to having a reduced sample size.

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