822
Monday - 642; Tuesday- 723; Wednesday- 953; Thursday- ? ; Friday- 662; Saturday- 813; Sunday- 612
The first digit= number of letters, the second digit= formed from the first letter's alphabet order and equal to sum of digits if 2-digit number, the third digit= number of vowels
M=13=1+3=4, T=20=2+0=2, W=23=2+3=5, F=6, S=19=1+9=10=1+0=1
Thursday=> 8 letters, T=20=2+0=2, U+A=2 --> 822