Hi,
Thank you for posting your query,
Please refer to the following steps to schedule the Backup according to your requirements.
1. Click on Backup or open Backup job properties.
2. Click on Schedule -> Run according to schedule.
3. Select the Effective date.
4. Click on Days of month, select Checkbox option "Last"
(This will run backup job on every last day of every month).
About the exclude:
5. Click on Excludes and select the dates manually.
At the end click on Summary option to check the Backup schedule. Backup exec will perform backup job the date(s) which are marked in Bold.
We hope this information helps, if you have any other questions please respond to the post and we will be more than happy to assist you further.