I decided to highlight it. My reasoning (which I'm not convinced about) was as follows. If the woodchuck sees the 6-letter word, then nothing changes - it's still the same time of year that you've highlighted interrupting the theme word. The second definition in Chambers is about a day on which things seem to happen in exactly the same way as on the previous day. If the 6-letter word is seen, then things today are the same as they were yesterday - still the same season. That's "more" consistent with the 2nd definition than not highlighting, which would assume a change.
Bit, err, convoluted. Other options, which I rejected, were 1a, and the letters in SUN and MON which have identical trajectories in the R-H side of the grid. There are quite a few areas in the grid with the letters for THINGS all connected, but that didn't get me anywhere either.