Any day can be reduced to a single number between 1 and 9.
For example: the day December 12. will end up to number 3, as 12 => 1 + 2 = 3. The day January 5 becomes 5, and 23 of February is also 5, as 23 => 2 + 3 = 5.
According to Cheiro have days 1, 10, 19 and 28 same energy as the sum of the digits of these numbers all become 1.
In other words, we are using the cross sum of your birthday
Please note, that the number for the month are not used here.