Factors that Obligate a Kohen to Go Up
There is a mitzvas asei for a Kohen to give a bracha to the tzibbur, as we mentioned. If a Kohen does not go up, he throws away a mitzvas asei, but that is only if three conditions, to be explained, are fulfilled and he does not go up. The three conditions are:
Called Out Invitation for the Kohen to Come Up
Calling “Kohanim!” A Kohen must go up when he is called up with the call of “Kohanim!” in shul. This is borne out by the Targum of the posuk “אמור להם,” which is “כד יאמרון להון” [“when they tell them”]. Meaning, the Kohanim are only obligated to give a bracha when they are told to (גמ' סוטה דף ל"ח ע"ב, תוס' ברכות דף ל"ד, שו"ע סי' קכ"ח ס"ב ומ"ב סק"ט).
Told to go up. Similarly, if someone tells a Kohen to go up, he must go up (שו"ע ס"ב, מ"ב סק"י ). However, some hold only a tzibbur or shliach tzibbur, not another individual, is able to obligate a Kohen (סידור היעב"ץ, מאירי מגילה דף כ"ד ע"ב הובא בביאה"ל סוף ד"ה או ונשאר בצ"ע ).
Told to wash his hands. Similarly, if the one washing the Kohanim’s hands tells a Kohen to come wash his hands, it is considered as though he was called up for Birkas Kohanim, and he is obligated to go up (מ"ב שם ). Here, too, some argue (as above, 8).
Only If He Was in Shul
Another condition for obligating Birkas Kohanim d’oraisa is that the Kohen must be in the shul when the Kohanim are called up or when he was told to come up or wash his hands, as mentioned (שו"ע ס"ב, הר"ן על הרי"ף מגילה פ"ד הובא בביאה"ל ד"ה אם היה).
Leaving His Place During the Tefilla of רצה
Another condition is that the Kohen must “uproot his feet,” i.e., begin moving forward to go up, at some point during the shliach tzibbur’s tefilla of רצה. רצה is called “Avoda” (גמ' סוטה דף ל"ח ע"ב), and they did Birkas Kohanim in the Beis HaMikdash before completing the avoda. Thus, Chazal established that in tefilla, too, the Kohanim should go up before the end of the bracha of Avoda, i.e., רצה (מ"ב סקכ"ו).
Did not begin moving before רצה. Some poskim say if a Kohen did not begin moving from his place during רצה, he is not obligated to go up even if he is called up or told to go up after the tefilla of רצה is completed, and he does not throw away a mitzvas asei (מג"א סק"ב, לקט יושר עמ' כ"ו, מ"ב סקע"ד בדעת המחבר ס"ח). Other poskim are unsure whether he has a chiyuv if he is called up if he did not begin going during רצה (א"ר סקט"ז בשת תשו' מהר"ם מינץ סי' י"ב הובא במ"ב סק"ט). Thus, in practice, if a Kohen did not begin going up during רצה, he should make sure to leave the shul before the call of “Kohanim!” and he should not go up for Birkas Kohanim at all so that he does not potentially throw away a mitzvas asei. Another reason for this is so that people do not say there is a problem with his Kohen status, as people do not know he did not move his feet somewhat during רצה (מ"ב סק"ט).
Mitzva for Kohen to Give Bracha in All Cases
Even when a Kohen does not have a chiyuv to do Birkas Kohanim and will not throw away the mitzvas asei by not doing it, e.g., in the above cases, he still has a mitzva to give the bracha. He should make an effort and strive to give the bracha when he can and not avoid fulfilling this important mitzva of giving a bracha to the tzibbur.
Once a Day
After a Kohen does Birkas Kohanim once, he is yotzei his chiyuv d’oraisa that day. Thus, even if he is told to go up again at a different tefilla that day, e.g., he did Birkas Kohanim at Shacharis and then was asked by a different Shacharis minyan to go up, he does not throw away the mitzvas asei if he does not go up (שו"ע ס"ג ). However, he still has a mitzva to go up a second time and he can make a bracha on Birkas Kohanim a second time (מ"ב סקי"א).
Different tefilla. Although there is no chiyuv d’oraisa to do Birkas Kohanim more than once a day, Chazal established that they should do Birkas Kohanim at Shacharis and also Mussaf the same day, and on Yom Kippur, at Ne’ila too, when it is in the same shul (שו"ע סי' קכ"ט ס"א ).
Already did Birkas Kohanim alone. Some say if a Kohen first does Birkas Kohanim alone and then has the opportunity to do it again with another Kohen, it is commendable for him to do it again with another Kohen. This is because he was not yotzei his chiyuv d’oraisa the first time according to some opinions (above 3), and now he has the opportunity to do Birkas Kohanim with another Kohen, fulfilling his chiyuv d’oraisa according to all opinions (קצות השלחן סי' כ"ג בדה"ש סקמ"ז).
