How to determine frequency for seasonal decomposition and other questions

I'm still a beginner in time series forecasting and I'm trying to decompose my data, but it returns ValueError: You must specify a freq or x must be a pandas object with a timeseries index with a freq not set to None , i already have a timeseries index and it's freq is None, my questions are

1. If i need to pass the freq parameter, how to know what numbers should i pass? I already read this but i still don't understand, if this is my date values, what freq should i pass? and why?

timeseriesDF.index
Out[107]: 
DatetimeIndex(['2019-01-01', '2019-01-01', '2019-01-01', '2019-01-01',
               '2019-01-01', '2019-01-01', '2019-01-01', '2019-01-01',
               '2019-01-01', '2019-01-01',
               ...
               '2019-03-30', '2019-03-30', '2019-03-30', '2019-03-30',
               '2019-03-30', '2019-03-30', '2019-03-30', '2019-03-30',
               '2019-03-30', '2019-03-30'],
              dtype='datetime64[ns]', name='date', length=1000, freq=None)

2. Is there anyway to solve the error besides passing the freq parameter?
3. Why does the freq seems like a must in seasonal decomposing?