Yom Kippur Riddle

Only one day in the year

Doing this during the year is forbidden.

This עבירה is so severe, that in some cases

you put the person in חרם.

According to some Poskim, he is חייב מיתה

(בידי שמים)

Only one day in the year, on Yom Kippur,  not 

only is it permitted to do, but some Poskim 

consider it commendable.

This is the only thing that is allowed on Yom Kippur 

and אסור all year*. 

????

*(Saying  Boruch Ssheim   K’vod Malchuso , aloud  is also done on

 Erev Rosh Chodesh., Yom Kippur  Katan, in most places)