I struggled with this too. The best understanding I've been able to come to is this:
1. Every month's item number can be calculated from the preceding month's item number by the method given. I.e. add the number of days in the preceding month to the preceding month's item number then take mod 7.
2. For January we take it that there is no preceding month. Therefore the numbers in such a calculation are all zero, so we end up with zero.
3. For later months you don't need to start at January and go month by month to the month you want; you can start at a month with a start- or end-vowel and use the shortcut that such a month's item number is ten minus the month number.
4. For the purposes of 3, 'y' does not count as a vowel.
1. Every month's item number can be calculated from the preceding month's item number by the method given. I.e. add the number of days in the preceding month to the preceding month's item number then take mod 7.
2. For January we take it that there is no preceding month. Therefore the numbers in such a calculation are all zero, so we end up with zero.
3. For later months you don't need to start at January and go month by month to the month you want; you can start at a month with a start- or end-vowel and use the shortcut that such a month's item number is ten minus the month number.
4. For the purposes of 3, 'y' does not count as a vowel.
Or maybe it's easier just to learn them!