Notice the use or non-use of the comma in the following sentences.
a. When I first visited Gandhi in 1942 at his ashram in Sevagram, he told me what happened in Champaran.
b. He had not proceeded far when the police superintendent's messenger overtook him.
c. When the court reconvened, the judge said he would not deliver the judgment for several days.

Explanation

a. In this sentence, the comma is used after a long introductory phrase.
b. Essential clauses do not require commas. In this sentence, the clause 'when the police superintendent's messenger overtook him' is an essential clause because it provides essential information. Hence, a comma is not required in this sentence.
c. In this sentence again we have an introductory clause which provides extra information. The second half of the sentence can stand alone and, therefore, is separated from the introductory clause with a comma.