agendus

Etymology
Substituting the missing present passive participle (gerundive) of.

Participle

 * 1) which is being done or made
 * 2) which is being driven

Declension
Necessity usually pertains when the gerundive comes with a conjugated form of esse, e.g.:
 * 1) Dies natalis agendus. - "Your birthday (which is) being celebrated."
 * 2) Dies natalis agendus est. - "Your birthday has to be celebrated."