There are two ways:
The first way is to choose a pre-made transparent grid. Open the Main tab. Click Month Style to open the grid editor.
You can find several transparent grids under the Classic category. Select the one you like and click OK to apply it.
The other way is to select transparent background in Template Editor. Choose Background, then set the row boxes to Transparent. Don't forget to click OK to apply the changes.
Here is the result: