This is one of those widespread myths that remains relatively undebunked. Probably because the answer to whether daddy-longlegs are venomous or not is not a simple yay/nay. Although it’s not far off.
There are in fact two creatures (sometimes three) that are most commonly identified as daddy-longlegs. Of these two, only one is actually a spider. The non-spider version belongs to its own Order (biology grouping term) and eats decomposing foliage. Entirely harmless unless you eat 10 of them, in which case you may have a sore stomach for a while.
The daddy-longlegs spiders on the other hand do have fangs. However due to their disinclination to actually bite and a couple of ethics roadblocks, very little research has been done into their venom. To be honest, we’re not even sure if these guys can even bite into human skin at all. So the guys over at Myth Busters (note- not a reliable source!) claim to have had a daddy-longlegs spider bite them on the arm and to everyone’s disappointment, inflict only mild and brief irritation.
So chill out. The oddly named daddy-longlegs aren’t gonna get you in the night. However there are plenty of other venomous animals in your pretty little garden that might…