I divide a year into 3 so month 4: April is ⅓ of a year, month 8 August is ⅔ of a year, then September, October, November December is the last ⅓ of the year.
⅓ : January, February, March, April
⅔ : May, June, July, August
Last ⅓ : September, October, November, December
The first ⅓ of the year is about slowly warming up and getting brighter.
The second ⅓ of the year is about being warm and bright with long days.
The last ⅓ of the year is about dimming and cooling to a frost with darker days.
The Alan-week is 10 days but it doesn't quite fit because some months have 31 days and February has 28 or 29 days on a leap year. April has 30 days so day 1 to 10 is the first ⅓
day 11 to 20 is the second ⅓
and day 21 to the last day is the last ⅓.
10, 20,30 ish, Like that.
Day 21 to the end varies by month. Some months the last ⅓ of the month is 10 days others it is 11 days and on February it is 8 days or 9 days on a leap year.
My son was born in the second third of April, I call that the middle of spring.