The Server poling history dialog lists recent polls for a server.
You reach this dialog from the main window / Server Sets tab by selecting a server set in the left list, expand the details for a server (right pane of content area), and clicking the "Polling history..." button.
Note: In the Options dialog / Server polling section, you can configure Simple Failover to "Only record poll result when server status changes", and "purge entries older than ___ days" which reduces the number of events listed here.
Note: The polling history is stored in a Microsoft SQL Server Compact database. The database file "poll-history.sdf" is stored in the application data directory (typically "C:\ProgramData\JH Software\Simple Failover").