it usually comes in the weekend after the ads start. if they have the kilo bags of raw king prawns part shelled, get some of those too, they are wonderful. Mine had frozen crabs at christmas which were also well worth the price.
Lidl's lobsters tend to be seasonal, coming in for Christmas. (I've got four still in my freezer - one scheduled for this weekend.) In the past they've beeen on special offer at some time after New Year, but I haven't seen that this year, (tho' I may have missed it). Cheaper than Asda's too.