When it comes to istikhara, a person hesitates between two matters, between two equal options.
Whether it is about obligation and the conflict in performing them, so a person cannot do both at the same time, or also when it comes to recommended acts.
Or also regarding benefit and purpose, or also regarding permissible things that a person cannot combine, or one permissible thing where a person hesitates between doing it or leaving it.
So we say this is the case. As for the istikhara a person does about whether to do something or not, and that is in matters that are legislated, we say this is different.
But he might do istikhara in some of its details, for example, whether to do it now or after a day or two, or something like that. We say for this kind of istikhara, there is no problem.
Except for what evidence shows should be done immediately. For example, if zakat becomes obligatory on a person, he must pay it right away; he should not say, "I will do istikhara about whether to pay it today or tomorrow."
Likewise, regarding Hajj, if someone seeks guidance about performing it this year or the next, we say he can do it as well. Is it among the matters of needs? We say it is of the same kind; it is among the needs for which a person asks his Lord.