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Without giving too much thought, you could use a $project stage to get the specified day.
Untested snippet
specified_day = "monday"
project_stage = { "$project" : { today : "$schedule." + specified_day } }
collection.aggregate( [ project_stage ] )
But the above will give all schedule.monday array. Using $filter allows you to only keep the appropriate elements.
To filter an array, you need https://www.mongodb.com/docs/manual/reference/operator/aggregation/filter/.
$filter : {
"input" : "schedule." + specified_day ,
"as" : "filtered" ,
"cond" : [ ]
}