It is said that there are some basic rules for correct punctuation as below. Commas are usually used; 1) when several kanji characters are written in succession, 2) when the subject is long, and 3) after conjunctions such as そして (SOSHITE/and) and でも(DEMO/but), etc.
However, the most important thing about using commas is to make the sentence easier to read. The comma in the example sentence was used for readability.
ことし、あなたのおかあさんのたんじょうびはなんようびでしたか。 (KOTOSHI, ANATA NO OKAASAN NO TANJOUBI WA NANYOUBI DESHITAKA) = On what day of the week was your mother’s birthday this year?
It would be a little hard to read if there was no comma between ことし (KOTOSHI/this year) and あなた(ANATA/you).