The color scale hugely amplifies minor differences, see my other comment. In the dataset, 15 years (2000-2014) are represented and the weekly cycle is therefore present. This could have been mitigated by using a 28-year dataset. Here is how often each month started on a given day in the dataset. We don’t have colored text so I used emoji.
01/ | JAN | FEB | MAR | APR | MAY | JUN | JUL | AUG | SEP | OCT | NOV | DEC |
---|---|---|---|---|---|---|---|---|---|---|---|---|
SUN | 2➖ | 2➖ | 1🔻 | 3⚠️ | 2➖ | 3⚠️ | 3⚠️ | 2➖ | 2➖ | 2➖ | 1🔻 | 2➖ |
MON | 2➖ | 1🔻 | 2➖ | 2➖ | 2➖ | 1🔻 | 2➖ | 2➖ | 3⚠️ | 3⚠️ | 2➖ | 3⚠️ |
TUE | 3⚠️ | 3⚠️ | 2➖ | 3⚠️ | 3⚠️ | 2➖ | 3⚠️ | 2➖ | 1🔻 | 2➖ | 2➖ | 1🔻 |
WED | 2➖ | 2➖ | 2➖ | 1🔻 | 2➖ | 2➖ | 1🔻 | 3⚠️ | 2➖ | 3⚠️ | 2➖ | 2➖ |
THU | 2➖ | 2➖ | 3⚠️ | 2➖ | 3⚠️ | 2➖ | 2➖ | 2➖ | 2➖ | 1🔻 | 3⚠️ | 2➖ |
FRI | 1🔻 | 3⚠️ | 2➖ | 2➖ | 1🔻 | 3⚠️ | 2➖ | 3⚠️ | 2➖ | 2➖ | 2➖ | 2➖ |
SAT | 3⚠️ | 2➖ | 3⚠️ | 2➖ | 2➖ | 2➖ | 2➖ | 1🔻 | 3⚠️ | 2➖ | 3⚠️ | 3⚠️ |
Here is how likely any date is to be Monday-Friday in the data (out of 15):
How often Mon-Fri | JAN | FEB | MAR | APR | MAY | JUN | JUL | AUG | SEP | OCT | NOV | DEC |
---|---|---|---|---|---|---|---|---|---|---|---|---|
01/08/15/22/29 | 10🔻 | 11➖ | 11➖ | 10🔻 | 11➖ | 10🔻 | 10🔻 | 12⚠️ | 10🔻 | 11➖ | 11➖ | 10🔻 |
02/09/16/23/30 | 11➖ | 12⚠️ | 12⚠️ | 10🔻 | 11➖ | 11➖ | 10🔻 | 11➖ | 10🔻 | 10🔻 | 12⚠️ | 10🔻 |
03/10/17/24/31 | 10🔻 | 11➖ | 11➖ | 10🔻 | 10🔻 | 12⚠️ | 10🔻 | 11➖ | 11➖ | 10🔻 | 11➖ | 11➖ |
04/11/18/25 | 10🔻 | 10🔻 | 11➖ | 11➖ | 10🔻 | 11➖ | 11➖ | 10🔻 | 12⚠️ | 10🔻 | 11➖ | 12⚠️ |
05/12/19/26 | 11➖ | 11➖ | 10🔻 | 12⚠️ | 10🔻 | 11➖ | 12⚠️ | 10🔻 | 11➖ | 11➖ | 10🔻 | 11➖ |
06/13/20/27 | 12⚠️ | 10🔻 | 10🔻 | 11➖ | 11➖ | 10🔻 | 11➖ | 10🔻 | 11➖ | 12⚠️ | 10🔻 | 11➖ |
07/14/21/28 | 11➖ | 10🔻 | 10🔻 | 11➖ | 12⚠️ | 10🔻 | 11➖ | 11➖ | 10🔻 | 11➖ | 10🔻 | 10🔻 |
You can see for example that August 01/08/15/22/29 is 10-20% more likely to be a workday than other days, which corresponds to the extra births. Feb 29 cannot be read directly from this table; its value is 3 out of 4 (2000: Tue, 2004: Sun, 2008: Fri, 2012: Wed).
As for your question: every month has approximately the same strength of the weekly cycle, it’s just that the prevailing colors in July-September show them the most. August’s is more visible because it does not have the disrupting July 4 and September 11.
Don’t forget the weekdays. The total births in the dataset by weekday are:
SUN
|||||||||||||||||||||||| 5886889MON
||||||||||||||||||||||||||||||||||||| 9316001TUE
||||||||||||||||||||||||||||||||||||||||| 10274874WED
|||||||||||||||||||||||||||||||||||||||| 10109130THU
|||||||||||||||||||||||||||||||||||||||| 10045436FRI
||||||||||||||||||||||||||||||||||||||| 9850199SAT
|||||||||||||||||||||||||| 6704495