The following text was written by Ruth Schreiber.
שולחן ערוך אורח חיים רפ הלכות שבת תשמיש המטה בשבת
א תשמיש מתענוגי שבת הוא לפיכך עונת תלמידי חכמים הבריאים מליל שבת לליל שבת
Shulchan Aruch, Orach Chaim Chapter 280 Marital Relations on Shabbat
Marital relations are one of the pleasures of Shabbat. Therefore scholars should have marital relations with their wives on Friday night.
This Jewish law (halachah), codified by Rabbi Yosef Karo in the 16th century in his major work “Shulchan Aruch”, is an integral, if private, side of orthodox Judaism. The term ‘scholars’ is interpreted to include all white-collar workers, as opposed to manual labourers for whom the halachah is different (Orach Chaim Chapter 240).
Just as a woman readies herself for the Sabbath by preparing food, challot and candles to light, so she must prepare herself, to the extent that she needs to do so and when she is not nidah (ritually unclean), for intimacy with her husband at the end of the day. After the meal is served and cleared, the guests leave and the children are asleep, then comes the time for the fulfilment of this mizvah.
In my Mizvah Night 1, I make overt reference to this hidden agenda of Friday nights. I have baked challot in the shape of breasts and put lipsticks in place of candles in the Sabbath candlesticks.
In my Mizvah Night 2, I have embroidered the relevant part of the text of Orach Chaim in English and Hebrew in gilt thread, and then cut out the words and attached the appliqués to gilt oversized candlesticks, thus making explicit one of the hidden meanings associated with Friday night candle-lighting.
Friday night really is often colloquially and half-jokingly called “mizvah night”.