crontab seems to support declaring variables upfront and substituting them
so this works for me:
SHELL=/bin/bash
log=/var/log/crontab/crontab.
dom=/bin/date +%a.%Y%m%d
ymd=/bin/date +%Y-%m-%d
and use that in
0 12 * * * wget https://example.com/example\_$($ymd)\_file.ext >> $log$($dom).log 2>&1