One feature on LiturgyTools.net is suggested free-use hymns for Sundays and feast-days in the Catholic liturgical calendar. These are organised based on the "name" for each day in the liturgical calendar, rather than the ordinary calendar - so they have titles like "21st Sunday of Ordinary Time, Year C".
But recently several people have asked questions like "which Mass is on 25 August?"
To know which liturgy is to be used on a given day, you need to look at a liturgical calendar. This shows which "church date" applies on each "calendar date", by showing a mapping between the liturgical and temporal calendars.
In the Roman Catholic church, the "church calendar" is centrally managed by the Vatican. Currently this is based on the 1969 document "General Norms for the Liturgical Year and the Calendar". The full text of this is available here, and a summary of the structure of the liturgical year is here.
But local leaders (usually the bishops conference in each country) must adapt this worldwide calendar to meet their particular needs and local celebrations. For example, in 2013, St Patrick's day (17 March - which is also a major secular holiday) fell on a Sunday so the Irish bishops had to decide how to handle this. American bishops sometimes face similar challenges with Our Lady of Guadelope's feast day. And local needs change over time, as particular devotions become more popular, or wider-church events affect the local calendars. So in most countries, the local bishops' conferences issue a new local liturgical calendar (technically called an Ordo) each year.
So - the first step in finding out what Mass is on the 25th of August is to look up the local liturgical calendar for your country. Lists of the Roman Catholic liturgical calendars for a variety of English speaking countries are available here.
Next, you need to check for any local feasts that may be specific to your diocese (ie not country-wide), too. Check your diocesan liturgy office for details - these are generally well know at local leve.
Also members and associates of some religious orders also need to consult their "family" calendars too, as there are some feast days that are particular to them.