You can establish the current direct debit date by logging in to My Virginmedia and going to the My Bills section where there is a link that leads to this page which advises your direct debit date and the process for changing it:
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Thanks for taking the time to query your Virgin Media account and the Direct Debit, we appreciate you taking the time to raise this and welcome to the forums.
Have you been able to follow the steps advised by Ernie_C in his post? It's also good to know that we do send bills to customers for their next bills roughly 10 working days before ewe request the money from their chosen bank so it may be that you have received a bill but the Direct Debit isn't due until 10 working days later (weekend depending)