Understand the convention for writing dates in Gaelic which differs from that used in North America and the United Kingdom.
In Gaelic the date is written literally as "the xth day of [the month]." For example, March 18th in North America would be written as 18th day of March in Gaelic.
Understand how to write the ordinal numbers (first, second, third) in Gaelic. Here are some examples, but you can find translations for all numbers through the link in the resources section.
First: a' chiad
Second: an dàrna
Ninth: an naoitheamh
14th: an ceathramh deug
21st: an t-aonamh ar fhichead
31st: an t-aonamh deug ar fhichead
Take note that the noun splits the ordinal phrase as in the translation of "the 31st day" which is "an t-aonamh latha deug ar fhichead" where "latha" means day. Also, there are two ways of writing numbers depending on the gender of the subject so check the translations carefully.
Understand how to translate months from English into Gaelic. Note that all months from January to August are masculine and the remaining months feminine. There are alternative names for some months but those listed below are the most commonly used:
Am Faoilteach: January
An Gearran: February
Am Màrt: March
An Giblean: April
An Céitean: May
An t-'gmhios: June
An t-Iuchar: July
An Lùnasdal: August
An t-Sultain: September
An Dàmhair: October
An t-Samhain: November
An Dùbhlachd: December
Translate the date you wish into Gaelic using the words you have translated and using the convention indicated in step one. You will also need to know the Gaelic word for day which is "latha" and "of" which is "dhen."
For example the 16th of March translates as "An siathamh latha deug dhen Mhàrt."