You can plant cucumbers outside when the weather is consistently warm and there is no risk of frost. Specifically:

• Sow cucumber seeds directly outside in late May or early June when soil and air temperatures have warmed enough, especially if night temperatures are reliably above 12°C (53°F) to 15°C (59°F).
• If you started cucumber seeds indoors, transplant seedlings outside from early June onwards after hardening them off and when outdoor temperatures are stable and warm (daytime around 21°C/70°F and nighttime above 15°C/60°F).
• Avoid planting cucumbers outdoors too early; wait until all risk of frost has passed and nights are warmer.
• For best growth, choose a sunny, warm, sheltered spot or grow near a south-facing wall to retain heat.
• Cover young plants with cloches or fleece if needed to keep soil warm and protect from cold at the start.

In summary, the safest window to plant cucumbers outside is from late May to early June depending on your local climate and temperature conditions.