RDS MySQL instance is unavailable, probably during binlog rotation
Your graph shows a binary log purge launched at approximatively 4am the 23th.

You can search for a query like:

PURGE BINARY LOGS BEFORE '2014-04-23 04:00:00';
-- or
PURGE MASTER LOGS BEFORE '2014-04-23 04:00:00';


If you are sure that your MySQL is a standalone instance, there is no know issues with the PURGE BINARY LOGS statement and I see that your binary log is 1.5Mb so i'm sure your system can manage that purge without pain. In a Master/Slave infrastructure, you can purge binary logs before they been "played" on slaves, the result is the slaves replication goes down (with impact on the apps).

Max.