, after Brutus has resolved to join the conspiracy against Caesar and met with the chief conspirators, his wife Portia enters and demands in the name of the bond between them to be informed of what has been troubling her husband recently:

Within the bond of marriage, tell me, Brutus,
Is it excepted I should know no secrets
That appertain to you? Am I yourself
But, as it were, in sort or limitation,
To keep with you at meals, comfort your bed,
And talk to you sometimes? Dwell I but in the suburbs
Of your good pleasure? If it be no more,
Portia is Brutus' harlot, not his wife.

Brutus is moved by this, and still more when she reveals that she has wounded herself in the thigh to demostrate her resolution and constancy, but he still keeps the secret of the conspiracy. Nevertheless, he promises to reveal everything to Portia when the right moment comes:

Portia, go in awhile,
And by and by thy bosom shall partake
The secrets of my heart.
All my engagements I will construe to thee,
All the charactery of my sad brows.
Leave me with haste.

It appears that he kept his promise soon after the close of the scene, since in Act II, Scene 4, Portia is in a passion of apprehension about something that will happen between Brutus and Caesar when the two meet, something that she must keep secret:

O constancy, be strong upon my side!
Set a huge mountain 'tween my heart and tongue!
I have a man's mind, but a woman's might.
How hard it is for women to keep counsel!

Portia's evasions before the people around her, and her panic at the words of the soothsayer, "Why, know'st thou any harm's intended towards him [Caesar]?", seem clearly to  indicate that she knows by that point that Caesar is to be murdered.

Thus, the promise that Brutus makes to Portia is to tell her about what has been troubling him -- the conspiracy -- and it appears from Portia's behavior that Brutus kept his promise before the assassination was carried out.