Hello

Should we say:
"with
a frequency of two lessons par week"
or
"with
the frequency of two lessons par week"?
I have googled the expressions and I have found both...
My sentence is:
"Mr Brown gave me French lessons from September to July with a/the frequency of two lessons par week".
As we have 'of' I think we should say 'the', but it is not always the case...
Thank you for your help!