meta_key value should be name of custom field. I doubt you have fields named after each month, so it’s likely should be eventdate.
meta_compare should have value that specifies a comparison operator, or just be omitted and it will default to = in that case.
From this you can’t really query full event date on equals condition to month alone. I am not sure but as I see it you will have either to add separate field for month/year (without day) or do much more complex query with posts_where filter.
See Time Parameters, Custom Field Parameters in Codex for more documentation.