Grief is an emotional roller coaster. It is an up and down ride. If you have a couple of good days and then a bad one, don't think of this as a setback. This is normal for grief. Grief is a high stressor.
Stress affects our immune system.
This is normal and usually the anticipation of these days is worse than the actual day. Let people around you know that such a date is approaching and ask for their understanding and support. Thus, this is not the time to make major decisions or changes in your life like selling your home, moving to another city, or taking a new job. If possible slow down and allow yourself time to adjust to your loss. Avoid adding to your losses by making major adjustments to your life.