This method managed to get not just my own but all my children birth days wrong, also far to complicated in the decade - this way works!
Divide 81 by 4 discard remainder and add to 81
e.g. 20+81 =101 Mod7 gives you 3
Month figure - use the (similar to the other method but...)
Therefore 3+1+30 =34 Mod7 is 6 Saturday
Using their method you 30+0+3+0 which gave Mod 7 of 5 Friday, which is incorrect.