In order to successfully start the day and fiscalize invoices, you need to add a deposit every day. (How to create and fiscalize an invoice, see here.)
To add a deposit, you need to click on the Accounts icon and select Finance in the drop-down menu. When Finance section is opened, click on Start of Day button.
Clicking on the start of the day button will open a window for entering the deposit, which can be 0 or the deposit you actually have in the cash register.
After that, click on the confirm button to confirm the entry of the deposit for that day, after which you will receive a message that the deposit has been successfully entered, and you can then fiscalize invoices.
The end of the day itself does not formally exist. You can select End of Day in the same part where the deposit is added, but that is an internal command, it is not sent to the tax administration.
At the end of the day, you can print the Fiscalized Invoices report or the VAT report, depending on what is more suitable for bookkeeping. (located in the Invoices report group).
Also, by clicking on the Financial reports button, you can get a list of fiscal reports available in Montenegro.