Celosia (flower) is classed very tender in the cited extension guidance. The timing rules below are relative to your local freeze dates — enter a location to turn them into calendar dates.
The rules this site applies
Method
Frost-relative window
Rule & reconciliation
Start seeds indoors
4–2 weeks before the median last spring freeze
University of Maryland's celosia production guide states seed is sown 4 weeks before the last frost date, which is the only frost-anchored figure found (Minnesota's 10-weeks guidance is anchored to the outdoor planting date, not to frost, so it cannot be used), giving a two-week sowing span opening 4 weeks before the median last freeze.
Transplant outdoors
from the median last spring freeze to 2 weeks after
University of Maryland says to sow or transplant celosia in the field after the danger of frost has passed and NC State places tender annuals after the threat of frost is past, so the window opens at the median last freeze; note Maryland's stricter companion rule of night temperatures consistently above 65 to 70 degrees F is not frost-anchored and cannot be expressed here, so the window's opening day is optimistic in cool-summer areas.
Direct-sow outdoors
from the median last spring freeze to 2 weeks after
The same University of Maryland sentence covers direct sowing — sow or transplant celosia in the field after the danger of frost has passed — so direct sowing opens at the median last freeze, with Illinois independently classing celosia a tender annual.
Variety assumption: Assumes Celosia argentea in its crested (cockscomb) and plumed forms; Maryland warns that exposure below 60 degrees F stunts plants and triggers premature flowering, so the frost-anchored window opens earlier than the temperature at which celosia actually thrives.
"Hardy annuals are the most cold-tolerant. They tolerate light frost and some freezing weather without being killed. In most cases, hardy annuals are planted in the fall or in the spring before the last frost date."