The farmers almanac says leeks can be transplanted outside in Toronto M4J 3T1 (Danforth/Pape area) after April 13 until April 24 — many other places say not to put them outside until end of April/beginning of May.
We seem to be in quite a micro climate here south of the Danforth — and nowhere to turn for definite guidance.
Thank you for contacting Toronto Master Gardeners.
The Toronto Master Gardeners website provides frost hardiness dates for vegetables that are direct seeded and information on vegetables to sow indoors , hardening off the vegetable, preparing the soil and location of planting. Growing Urban Vegetables a Toronto Master Gardeners Guide. Leeks are a biannual vegetable that prefers full sun and take at least 65 up to 120 days to maturity depending on the variety. Space 5 inches apart and plant as deep when planting in soil, Leeks are hilled up with soil as they grow to keep the stems white.
West Coast Seeds listed at the bottom of the Toronto Master Gardener Guide provides a table of when to sow leek seeds indoor and when to plant outdoors. They have an estimated date of April 21st as the last frost date for Toronto. The leek seedlings are listed as being able to transplant in May after being hardened off. Added is the website PDF file for your perusal: Southern Ontario West Coast Seeds Planting Guides
For your information the map of hardiness zones by Municipality: Plant Hardiness Zone by Municipality
Happy Gardening