To start with , alert and report are two different knowledge objects in Splunk. An alert will be generated only when specific condition like number of results etc are met. Where as a report is the result of your query and can be viewed similar to a dashboard, it can also be scheduled as mail and the results will be mailed to you.
Assuming your question is about alerts, there are two ways in which you can set it up.
Run the query for the desired time range (in your case the complete month) and then on the top right corner of search bar click on save as alert and in that you will be required to give the schedule viz is when you want splunk to run the query for you (in your case 1st of every month)
Alternatively you can go to settings -> Searches, reports, and alerts in that new alert and provide the same info. Here also time range is your range for which you want splunk to run query and schedule is when you want it to run.
Coming to reports create the report in the same way, once you have created the report , find it in settings -> Searches, reports, and alerts and click on edit and schedule the report to run on the specific time you want it to run
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