By English Lavender, I assume you mean Lavandula augustifolia - it does well in Mediterranean countries, not sure if it has an upper heat tolerance, but will certainly thrive in temperatures up to 45 °C and likely higher. French lavender does well Zones 8-11, and the upper temperature in Zone 11 is around 50 °C.
What Lavender doesn't like is heavy, damp soil - it prefers gritty, light, free draining conditions. If you're growing in pots, then you will need to keep them watered. Regarding when to plant them, they are more heat resistant once established, but that's if you're growing them in the ground, so in pots, I don't think it matters. You can at least move them or provide some shade if the weather is extremely hot.