dates.md

You can use re module together with string manipulation to extract the dates easily

import requests
import re
import json

if __name__ == "__main__":
    texts = [
        'en los dias 3,06,8 ,9, 15 y 29 de diciembre de 2018.Por c',
        'n en: 2,04,05,8,9,10,11,14,15,22,24, y 27 de junio de 2018.Valor de',
    ]
    # select from the beginning of date-like text till the end of year
    pattern = r'\s*((\d+[\sy\,]*)+[\D\s]+20\d{2})'
    month_names = ['diciembre', 'junio']  # add others
    month_pattern = re.compile(f'({"|".join(month_names)})', flags=re.IGNORECASE)

    all_dates = []
    for item in texts:
        match = re.search(pattern, item)
        if not match:
            continue
        date_region: str = match.group(1)
        
        # find year
        year = re.search('(20\d{2})', date_region).group(1)
        
        # find month
        month_match = re.search(month_pattern, date_region)
        month = month_match.group(1)
        # remove everything after month
        date_region = date_region[: month_match.start()]
        # find all numbers, we're assuming they represent day of the month
        days = re.findall('(\d+)', date_region)
        
        found_dates = [f'{d}/{month}/{year}' for d in days]
        all_dates.append(found_dates)
    print(all_dates)
        

I don't know the month names in Portuguese?, but replacing those is a trivial task. output:

[['3/diciembre/2018',
  '06/diciembre/2018',
  '8/diciembre/2018',
  '9/diciembre/2018',
  '15/diciembre/2018',
  '29/diciembre/2018'],
 ['2/junio/2018',
  '04/junio/2018',
  '05/junio/2018',
  '8/junio/2018',
  '9/junio/2018',
  '10/junio/2018',
  '11/junio/2018',
  '14/junio/2018',
  '15/junio/2018',
  '22/junio/2018',
  '24/junio/2018',
  '27/junio/2018']]