We want to have a baseline of planing created automaticaly 1 time per month.
Is this possible ?
Yes, this should be possible.
In your project workflow, set up a timeout step that will fire every 24 hours. In this step, you can run a SQL Query that leverages the date from DB (SYSDATE) to check if you're on the right day of the month on which you want to capture your baselines.
Since the workflow timeout is not extremely precise (it depends on the timeout reaper background service), you might set it to run every 23 hours and insert a row with current month in a custom table to indicate that your snapshots have been taken for this month - and check if a row already exists for that month before taking the snapshot, and skip if it already exists. If you need the snapshot to be taken at the same time of the day, just have your timeout fire more often, but don't set the value too low as it may create quite a lot of workflow-related data in your DB.