The visitor center at Hoh is weekend-only until June. Is this because the forest is no fun or hard to walk in, or because not very many people show up? I will be close enough to consider a trip in late April.
I think that it is mostly due to the fewer people, as there are multiple visitor centers throughout the park, some open every day. However, in the winter season, it rains a lot in the forest so this is less comfortable. As it is still open on weekends, it is not impracticable, just less popular.
If you visit in the lower season, be careful as some roads may be closed and weather can be a problem. Make sure you check the current conditions before visiting the park.