Is specifying a particular day or month to anticipate hearing desired news or the fulfillment of a wish, and attempting to imagine the realization of this wish, considered a good assumption about Allah? And how can one instill a good assumption in the heart?

If a servant of Allah wishes for something from Him and supplicates for it without transgression, and it occurs to him that Allah will answer him as he wishes and when he wishes, then he has a good opinion of Allah. The believer must know that thecription of supplication is not limited to fulfilling the apparent request; Allah may avert evil from him through it, or store it for him until the Day of Resurrection. This helps in having a good opinion of Allah.