The extra mini build this month is the Star Wars one, as it's part of the LEGO Star Wars events that are starting this weekend to mark the yearly Star Wars Day celebrations.
The arrangements for these free LEGO mini builds are the same as usual: they are held in LEGO stores from 4pm to 6pm, and are for children aged 6 to 14 only. You cannot buy the models separately, it's one per person, and the children are shown how to build the model in store then get to take home their creation for free.
You can't book places, and it's only while stocks last, so it's advisable to get their early if you can, just to avoid disappointment.
You can find details of these and other events on the LEGO Stores webpages*.