Strategy of chocolate company

It is the day (when) girls give chocolates to the boys they love.

We don't know the meaning of the St.

According to the effort of the chocolate industry, it is one of the most popular events in Japan, like Christmas.

We have another custom where girls to give chocolates to her boss or co-worker.

Today it is Saturday, so almost company is holiday. (I don't quite understand what it is you're trying to say here. Sorry. ^-^;)

It is possible to lose the sales of chocolate. (Do you mean something along the lines of the sales of chocolates decreasing after Valentine's Day? Sorry~)

The chocolate companies make "gyaku choko" to maintain the sales.

"Gyaku" means upside-down.

Usually the girl gives it to a boy, but there is another proposal that that the boy can also give it to the girl who he loves.

According Thanks to the efforts of the chocolate industry, it is one of the most popular events in Japan like Christmas. Comment: In English, "According to" is closer to "as claimed/asserted by" than "through the efforts of", which I believe is what you were trying to say. In English, thanks can be given in "gratitude" for something, but "thanks to" can also simply be an acknowledgement of some effort or effect.

Today it is Saturday, so almost most companyies is are on holiday. Comment: We don't normally refer to the weekends as "holidays" in English. A better translation of 休み would probably be "break" (in this case). Alternately, you could say "Today is Saturday, so most people will not be at work."

It is possible to lose that the sales of chocolate will be lower than normal (for Valentine's day). Comment: I think I understood you correctly. Were you suggesting that, because the ladies will not be buying chocolates for their bosses and coworkers, sales will be lower than in other years?