The best time to visit Negombo's west coast is from November to April, when the southwest monsoon has passed and the sea is calm. December and January are busy with European visitors escaping winter; prices rise and guesthouses fill up. The shoulder months of November and April offer good weather with fewer crowds.
The southwest monsoon arrives in May and blows through October, bringing rough seas and overcast skies. The beach becomes unsuitable for swimming, but the town functions normally — and room rates drop significantly. The fish market is active year-round regardless of season.
If you're just stopping overnight en route to the rest of Sri Lanka, the season matters less. The town's main draw — the canal, the fish market, the colonial quarter — is accessible in any weather.