There is no perfect month for every coast
Sri Lanka is warm almost all year, but rain and sea conditions shift with coast, altitude and monsoon. Elephant Tours’ month-by-month weather guide puts average temperatures at about 27°C on the coast and 16°C in the hills, with roughly 12 hours of daylight.
The two main monsoons are the south-west (May–September), which mainly affects the south and west, and the north-east (December–February), which affects the north and east.
How that maps to travel from Israel
December–March is the classic window for the south and west beaches. April is hot and also suits the hills, coinciding with the Sinhala New Year. May–September shift the better beach odds to the east (Pasikuda, Arugam Bay), with July–September also described as good for safari. October is the least settled month in that guide; November starts wetter and improves later in the month.
Use that as a planning frame, not a daily forecast.
A winter trip often follows a Kandy–tea-country–south-coast shape, such as the 10-day hills, safari and south-coast itinerary. A mid-year trip should usually change the coastal emphasis rather than copy a winter route. See the recommended itineraries for family and couple options.
If you are also considering an investment, contact Kenglobe Lanka. Holiday planning is handled by Elephant Tours.