I would like to ask about the correct approach in which the seasonal forecast model run should be compared with its relevant hindcast run(s). 

Let's consider the ECMWF forecast monthly data on a single level starting on 1st February 2020. I want to calculate, for instance, monthly anomaly of air temperature for May 2020. It is already available as the final product in CDS, I know, but I'd like to learn a method how this is done. What hindcast runs are used to calculate May 2020 temperature anomaly and what other steps follow? 

1) All hindcast runs starting from 1st February (1.2.1993, 1.2.1994, 1.2.1995 ..... 1.2. 2016) are considered. The individual ensemble members data for May 1993, 1994.... 2016 are taken and mean value over all these ensemble members and years is calculated and this is then used as the "hindcast climatology" for the month of May.

or

2) All hindcast runs starting from 1st of May (1.5.1993, 1.5.1994, 1.5.1995.... 1.5.2016) are considered and all others steps are the same as above.

or

3) None of these two options, it's done in a different way.... (how?)

Another question related to this is whether I could also download an ensemble mean value calculated over all hindcast members starting on the same day (e.g., mean value from all ensemble members of hindcast starting on 1.1.1993 etc.). Could I?

Thank you for any suggestions and advice.

Petr