1)
What is the meaning of "Hiskadshu"?
1.
Rashi: It means 'Prepare yourselves for punishment!' 1
2.
Moshav Zekenim: The seventy Zekenim will have only this prophecy tomorrow, that you will tell the people "Hiskadshu" (but Eldad and Meidad will not continue to have prophecy).
1
Rashi: As in Yirmiyah, 12:3. See Sifsei Chachamim.
2)
Why did Hashem say "Hiskadshu le'Machar" and not straighhtaway?
1.
Oznayim la'Torah: Perhaps they would do Teshuvah and the punishment would be averted.