These are two other possibilöities, but don't rely too heavily on google when you get an expression like this - I speak as an ENS, and I think in your context, your first entry is the more likely to have favour.

Mike :)

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Note added at 9 mins (2006-07-11 11:13:48 GMT)
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Much more common - 888 hits in Google

"next to last day of the month"

Tailrank - Guard to Miss Border Mission DeadlineAs of Thursday, the next-to-last day of the month, fewer than 1000 troops were in place, according to military officials in the four border states of Texas, ...
tailrank.com/posts/562949953702605/ Guard_to_Miss_Border_Mission_Deadline - 72k - Cached - Similar pages


AP: Guard to Miss Border Mission Deadline - Examiner.comAs of Thursday, the next-to-last day of the month, fewer than 1000 troops were in place, according to military officials in Texas, California, ...
www.examiner.com/a-164523~AP_ _Guard_to_Miss_Border_Mission_Deadline.html - 35k - Jul 9, 2006 - Cached - Similar pages


Chron.com | Guard to miss border deployment deadlineAs of today, the next-to-last day of the month, fewer than 1000 troops were in place, according to military officials in the four border states of Texas, ...
www.chron.com/disp/story.mpl/front/4013097.html - 31k - Cached - Similar pages


States say border force short of goal | Chicago TribuneAs of Thursday, the next-to-last day of the month, fewer than 1000 troops were in place, officials in Texas, California, New Mexico and Arizona said. ...
www.chicagotribune.com/news/nationworld/ chi-0606300122jun30,1,5591206.story?coll=chi-newsnationworld-hed - Similar pages


[PDF] resolution letter- giltedge unit holders-final3File Format: PDF/Adobe Acrobat - View as HTML
the next to last day of the month. (b). is to be paid monthly, ie, on the last day of the month or as soon as possible thereafter, by ...
jaunittrust.com/PDF/resolution_letter.pdf - Similar pages


Guardsmen Fail To Patrol Border As They Tend to Floods and Fires ...As of yesterday, the next-to-last day of the month, fewer than 1000 troops were in place, according to military officials in the four border states of Texas ...
www.nysun.com/article/35378 - Similar pages


USATODAY.comAs of Thursday, the next-to-last day of the month, fewer than 1000 troops were in place, according to military officials in the border states of Arizona, ...
www.usatoday.com/printedition/ news/20060630/a_capcol30.art.htm - 34k - Jul 9, 2006 - Cached - Similar pages


DailyBulletin.com - Guard misses border deadlineAs of Thursday, the next-to-last day of the month, fewer than 1000 troops were in place, according to military officials in the four border states of ...
www.dailybulletin.com/news/ci_3996546 - 38k - Cached - Similar pages


Guard to miss border deployment deadline (AP)As of today, the next-to-last day of the month, fewer than 1000 troops were in place, according to military officials in the four border states ...
www.democraticunderground.com/discuss/ duboard.php?az=view_all&address=102x2363721 - 15k - Cached - Similar pages


In Other News... for Friday June 30, 2006As of Thursday, the next-to-last day of the month, fewer than 1000 troops were in place, according to military officials in the 4 border states of Texas, ...
newstandardnews.net/content/ ion/index.cfm/days/2006-06-30 - 35k - Cached - Similar pages



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Note added at 10 mins (2006-07-11 11:14:44 GMT)
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In the U.S., at least, "next-to-last day" (without "the") is much more common that either choice you gave.

The day before the month ends

I don't think you can go wrong with penultimate.
If you google for 'penultimate day of the' you will find that this expression is used widely to indicate the last day but one of various time periods.