Historically, September was the month of the year where it would rain very heavily throughout Sri Lanka and so everyone avoided travelling in the country then. As time has gone by, the monsoon has arrived later and later in the year, so that now it tends to arrive in November.
This makes September a brilliant time to visit, boasting:
Good weather. Should there be rain it usually consists of short sharp showers.
Off season prices reflecting the old low season period.
Central, northern and eastern areas are not affected by the first monsoon so the weather there is ideal. This includes the cultural triangle, east coast, eastern hill country and lots of the wildlife parks. You can essentially do and see most of what Sri Lanka is famous for and the weather should be great.
https://evisa-lk.com/