which node will execute events in master-master setup
As far as I know, events in a MariaDB Galera Cluster must be configured to run on only one of the nodes. While CREATE EVENT is replicated to the other nodes, you have to choose which node to use for running the event scheduler: Execute SET GLOBAL event_scheduler = ON; on the node where you want events to run, or set it in your config file if you want it the setting to survive a restart. 

If that node is down, then there is no automatic failover for events, so if you'd like to run the events on another node, you'll have to enable the event scheduler on that node.

You could perhaps control the event_scheduler setting through software: a script could poll each node, and make sure the setting is enabled on one and only one node.