Everyone will tell you parsnip seed must be fresh. Everyone is right, and the reason is not the one usually given — it is not that the seed goes stale, it is that the seed is a winged papery thing that dries out in a packet faster than almost anything else you will sow.
March and April to sow. October to December to lift. Nothing in between, which is why the bed is committed for two thirds of the year.
Sow three seeds every 150 mm at 13 mm deep and thin to the strongest. This looks wasteful and is not: germination is genuinely poor, and the alternative — a continuous drill thinned later — disturbs the roots of the survivors.
Do not transplant. A parsnip moved at any stage forks, and a forked parsnip is a fine parsnip that takes four times as long to peel.
Frost improves them. Starch converts to sugar and a parsnip lifted in December tastes materially different from one lifted in October. Leave them in the ground and lift as you need them; they will keep there until the tops start away in February.