Q: When is the best time to do istighfar?
Ans: In the early morning hours (sahur).
Give yourself permission to be the brand new you by doing istighfar in the early morning!
Allah says in Surah Ali-Imran verse 3:17:
الصَّابِرِينَ وَالصَّادِقِينَ وَالْقَانِتِينَ وَالْمُنفِقِينَ وَالْمُسْتَغْفِرِينَ بِالْأَسْحَارِ ﴿١٧﴾
Which means:
"The patient, the true, the obedient, those who spend [in the way of Allah], and those who seek forgiveness before dawn. (17)"
And again in Surah Az-Zariyat verse 51:18:
وَبِالْأَسْحَارِ هُمْ يَسْتَغْفِرُونَ ﴿١٨﴾
Which means:
"And in the hours before dawn they would ask forgiveness, (18)"