Julie, our Miiverse guide says via the company's official Miiverse announcement thread that the service will undergo maintenance from Friday September 26th starting at 3PM to Tuesday September 30th ending at 1AM Pacific.
Here's the rest of Julie's message which you can find with a link for further details at the Miiverse page we've linked to below:
If you receive an error message such as 121-1503, 015-5003, 015-4503, or 115-4503, please wait a while before trying to use the service again. We sincerely apologize for the inconvenience. More information is available on the Network Maintenance Information / Operational Status page on Nintendo.com's Support page.