A show of solidarity is the only justifiable basis for the timing. I concur it had that potential. However, the speeches had less of a tone of “we will stand together no matter what this administration tries to do” and more of a tone of “FU Trump”.
As for marching against something after enactment, those protests are common and often effective. Plus, significant laws impacting large groups of Americans rarely pass overnight without great fanfare beforehand. Like we saw with Obamacare. Marches can be assembled as voting on signification issues are anticipated to occur and the message of the march is far clearer.