Ask any fan of Japanese anime or films how to say "bon appétit" in Japanese and they will immediately respond “itadakimass”, even if they don’t speak the language. In reality, it is a bit more complicated than that.
This "translation" is actually a shortcut made possible because, both phrases are, something we say before a meal. However, 頂きます itadakimasu does not exactly mean "enjoy your food" but rather "I receive" or even, by extension, "I ingest" (food and drink).
An expression of humility that expresses gratitude, itadakimasu is not a social nicety, but something rather more spiritual. Unlike the expression "bon appétit", it is not said to a group, and you cannot wish to others. Rather, it is something you say to yourself, so you can say it even if you eat alone.
It is said not only to the cook but also to those who have put the food on the table, even down to the foodstuffs... The host or the chef, if any, might also say 召上がれ meshiagare ("to get excited"), indicating that the meal is served.
One can make a slight bow of the head while saying itadakimasu, clasping the palms vertically while holding the chopsticks horizontally between both thumbs.
Additionally, there is no need to reply "thank you" (arigato), as this would make no sense.
During and after the meal
It is considered good form to voice your enthusiasm for your food using expressions like 美味しい "Oishii!" ("It's good!") and 甘い "Umai!" ("It's delicious!") after the first bite, as if you had never eaten anything so good.
The end of the meal follows the same process of politeness as at the start, but this time, you use the phrase ご馳走様(でした) "Gochiso-sama (deshita)" which literally means "It was a feast."